System and method for maintaining efficiency of a heat sink

ABSTRACT

A heatsink comprising a heat exchange device having a plurality of heat exchange elements each having a surface boundary with respect to a heat transfer fluid, having successive elements or regions having varying size scales. According to one embodiment, an accumulation of dust or particles on a surface of the heatsink is reduced by a removal mechanism. The mechanism can be thermal pyrolysis, vibration, blowing, etc. In the case of vibration, adverse effects on the system to be cooled may be minimized by an active or passive vibration suppression system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 15/648,065, filed Jul. 12, 2017, now U.S. Pat. No. 10,830,545,issued Nov. 10, 2020, which is a non-provisional of, and claims benefitof priority under 35 U.S.C. § 119(e) from U.S. Provisional PatentApplication No. 62/361,253, filed Jul. 12, 2016, the entirety of whichis expressly incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the field of heatsinks or devices thattransfer heat between a concentrated source or sink and a fluid, andsystems and methods for maintaining the efficiency of the heatsink andcleaning heatsinks.

BACKGROUND OF THE INVENTION

All references mentioned herein are expressly incorporated by referencein their entirety for all purposes.

A heat sink is a term for a component or assembly that transfers heatgenerated within a solid material to a fluid or gas medium, such as airor a liquid. A heat sink is typically designed to increase the surfacearea in contact with the cooling fluid or gas surrounding it, such asthe air. Approach air velocity, choice of material, fin (or otherprotrusion) design and surface treatment are some of the design factorswhich influence the thermal resistance, i.e. thermal performance, of aheat sink. See, en.wikipedia.org/wiki/Heat_sink.

Heatsinks operate by removing heat from an object to be cooled into thesurrounding air, gas or liquid through convection and radiation.Convection occurs when heat is either carried passively from one pointto another by fluid motion (forced convection) or when heat itselfcauses fluid motion (free convection). When forced convection and freeconvection occur together, the process is termed mixed convection.Radiation occurs when energy, for example in the form of heat, travelsthrough a medium or through space and is ultimately absorbed by anotherbody. Thermal radiation is the process by which the surface of an objectradiates its thermal energy in the form of electromagnetic waves.Infrared radiation from a common household radiator or electric heateris an example of thermal radiation, as is the heat and light (IR andvisible EM waves) emitted by a glowing incandescent light bulb. Thermalradiation is generated when heat from the movement of charged particleswithin atoms is converted to electromagnetic radiation.

Heat transfer is the exchange of thermal energy between physicalsystems. The rate of heat transfer is dependent on the temperatures ofthe systems and the properties and states of the intervening mediumthrough which the heat is transferred. The three fundamental modes ofheat transfer are conduction, convection, and radiation. Heat transfer,the flow of energy in the form of heat, is a process by which a systemchanges its internal energy. The direction of heat transfer is from aregion of high temperature to a region of lower temperature, and isgoverned by the Second Law of Thermodynamics. Heat transfer changes theinternal energy of the respective systems, and occurs in a directionthat increases the entropy of the collection of systems. Thermalequilibrium is reached when all involved bodies and the surroundingsreach the same temperature. Thermodynamic and mechanical heat transferis calculated with the heat transfer coefficient, the proportionalitybetween the heat flux and the thermodynamic driving force for the flowof heat. See, Daniel Arovas, Lecture Notes on Thermodynamics andStatistical Mechanics (A Work in Progress), Department of Physics,University of California, San Diego, Nov. 14, 2013.

The fundamental modes of heat transfer are: Advection (the transportmechanism of a fluid from one location to another, and is dependent onmotion and momentum of that fluid); Conduction or diffusion (thetransfer of energy between objects that are in physical contact);Convection (The transfer of energy between an object and itsenvironment, due to fluid motion); and Radiation (The transfer of energyby the emission of electromagnetic radiation in the infrared part of thespectrum).

Heat conduction occurs as hot, rapidly moving or vibrating atoms andmolecules interact with neighboring atoms and molecules, transferringsome of their energy (heat) to these neighboring particles. Conductiontends to be the most significant means of heat transfer within a solidor between solid objects in thermal contact. Heat transfer between theheat source and heat sink, as well as through the heat sink, areconductive transfer. Advection operates by transferring matter with itsthermal energy, over space. Convective heat transfer, or convection, isthe transfer of heat from one place to another by the movement offluids, a process that is essentially the transfer of heat via masstransfer, and usually combines effects of heat conduction within thefluid (diffusion) and heat transference by bulk fluid flow streaming.

Convective cooling is sometimes described as Newton's law of cooling:The rate of heat loss of a body is proportional to the temperaturedifference between the body and its surroundings, however convectivecooling sometimes deviates from this “law”. In general, convection isnot linearly dependent on temperature gradients, and in some cases isstrongly nonlinear.

radiative transfer between two objects is described by and T is theabsolute temperature (in Kelvin or Rankine).

Radiance or spectral radiance is a measure of the quantity of radiationthat passes through or is emitted. Radiant barriers are materials thatreflect radiation, and therefore reduce the flow of heat from radiationsources. The effectiveness of a radiant barrier is indicated by itsreflectivity, which is the fraction of radiation reflected. A materialwith a high reflectivity (at a given wavelength) has a low emissivity(at that same wavelength), and vice versa. At any specific wavelength,reflectivity=1−emissivity.

A heatsink tends to decrease the maximum temperature of the exposedsurface, because the power is transferred to a larger volume. This leadsto a possibility of diminishing return on larger heatsinks, since theradiative and convective dissipation tends to be related to thetemperature differential between the heatsink surface and the externalmedium. Therefore, if the heatsink is oversized, the efficiency of heatshedding is poor. If the heatsink is undersized, the object may beinsufficiently cooled, the surface of the heatsink dangerously hot, andthe heat shedding not much greater than the object itself absent theheatsink.

A heat sink transfers thermal energy from a higher temperature to alower temperature fluid or gas medium, by a process such as radiation,convection, and diffusion. The fluid medium is frequently air, but canalso be water or in the case of heat exchangers, oil, and refrigerants.Fourier's law of heat conduction, simplified to a one-dimensional formin the direction x, shows that when there is a temperature gradient in abody, heat will be transferred from the higher temperature region to thelower temperature region. The rate at which heat is transferred byconduction, q_(k), is proportional to the product of the temperaturegradient and the cross-sectional area through which heat is transferred:

$\begin{matrix}{q_{k}=={{kA}\frac{dT}{dx}}} & (1)\end{matrix}$

where q_(k) is the rate of conduction, k is a constant which depends onthe heat-conducting material, A is the surface area through which theheat is conducted, and dT/dx is the temperature gradient, i.e., the rateof change of temperature with respect to distance (for simplicity, theequation is written in one dimension). Thus, according to Fourier's law(which is not the only consideration by any means), heatsinks benefitfrom having a large surface area exposed to the medium into which theheat is to be transferred.

When dust settles on a heatsink, the area changes (typically increases,but by coating a microstructured surface, the area may decrease), andthe constant k will typically decrease, since the dust is not anoptimized heat transfer material, and often is a heat insulatingmaterial. The result is significant loss of heatsink efficiency.

Consider a heat sink in a duct, where air flows through the duct, andthe heat sink base is higher in temperature than the air. Assumingconservation of energy, for steady-state conditions, and applying theconvection-cooling law, also known as the Newton's law of cooling, givesthe following set of equations.

$\begin{matrix}{{\overset{.}{Q} = {\overset{.}{m}{c_{p,{in}}\left( {T_{{air},{out}} - T_{{air},{in}}} \right)}}},} & {(2),} \\{{\overset{.}{Q} = \frac{T_{hs} - T_{{air},{av}}}{R_{hs}}},} & {(3),} \\{{{{where}\mspace{14mu} T_{{air},{av}}} = \frac{T_{{air},{out}} + T_{{air},{in}}}{2}},} & (4)\end{matrix}$

and {dot over (Q)} is the first derivative of the thermal energy over

${time} - {\overset{.}{Q} = {\frac{dQ}{dt}.}}$

Using the mean air temperature is an assumption that is valid forrelatively short heat sinks. When compact heat exchangers arecalculated, the logarithmic mean air temperature is used. {dot over (m)}is the first derivative of mass over time, i.e., the air mass flow ratein kg/s.

The above equations show that when the airflow through or around theheat sink decreases, this results in an increase in the average airtemperature. This in turn increases the heat sink base temperature. Andadditionally, the thermal resistance of the heat sink will alsoincrease. The net result is a higher heat sink base temperature. Theinlet air temperature relates strongly with the heat sink basetemperature. Therefore, if there is no air or fluid flow around the heatsink, the energy dissipated to the air cannot be transferred to theambient air. Therefore, the heatsink functions poorly.

The fractal or branching architecture may be compelled by the thermaltransfer design, or other design constraint. For example, a fractalantenna may also serve as a heatsink, with the fractal features notcritically optimized as comparted to other designs with respect to heatshedding. See, Casanova, Joaquin J., Jason A. Taylor, and Jenshan Lin.“Design of a 3-D fractal heatsink antenna.” Antennas and WirelessPropagation Letters, IEEE 9 (2010): 1061-1064. See also, Dannelley,Daniel. Enhancement of extended surface heat transfer using fractal-likegeometries. Diss. The University of Alabama TUSCALOOSA, 2013; and Lee,S. R., Li, Z. G., Wang, B. G., Chiou, H. S., 2005, “An Application ofthe Fractal Theory in the Design of Heat Sink for Precision MeasurementInstrument,” Key Engineering Materials, 295-296, pp. 717-722.

If a heatsink is initially optimized, the accretion of dust at thesurface will de-optimize the air flows and heat conductivity of heatsinkfins, and also decrease efficiency on that basis.

Various methods have been proposed for removing dust from heatsink fins,including vibration (See, U.S. 20070058346; 20080121373; 20080121374;20090272404; 6,544,309; 5,566,377; 8,203,840; 8,400,766), air jets, andthe like.

There are various methods to reduce damage to substrates (e.g.semiconductors) while being subjected to vibrations (e.g. ultrasound)for purposes of cleaning (e.g. removing dust etc.). For example, thepower intensity of vibration may be lowered using an attenuator, etc.(See, U.S. Pat. No. 6,679,272; US 20060260638; WO2008086479A2.

The frequency of vibrations may be controlled to reduce the effect onnano-dimensioned structures on the substrate (US 20130206165). Moregenerally, the sensitivity of the structure to be protected to vibrationas a function of frequency may be determined, and the high sensitivityfrequencies may be avoided.

The directionality of waves may be controlled using constructiveinterference (U.S. Pat. Nos. 6,276,370; 7,614,406). The angle ofincidence of the vibrations onto substrate may be controlled (U.S. Pat.No. 7,238,085). That is, vibrations or shock waves from a transducerhave a propagation direction, as they travel along a vector path. As aresult, the vibrations may be cancelled by either active means, i.e. asecond transducer, or passive means, by causing self-interference. Ineither case, the vibrations at a point may be cancelled, while in otherregions, the vibrations may be significant.

It is noted that the vibrations used to facilitate cleaning may have lowdamage potential for the sensitive structures per se, but could causedamage as a result of resonances and constructive and/or destructiveinterference (U.S. Pat. No. 5,834,871; US 20050003737). Therefore, thestructures may be designed to avoid enhancement of the vibrationamplitude at or near sensitive structures, while potentially ensuringresonances and constructive interference at distal structures where thevibration action is intended.

Thermal interfaces that are elastomeric in nature may be used to isolatethe sensitive structures from vibrations (U.S. Pat. No. 6,002,588).Similarly, through-holes may be provided within the substrate in orderto dampen the vibrations (US 20080017219). More generally, a selectivethermally conductive vibration damping structure or material may beprovided disposed along the path of vibrations from a source to thethermal emitter.

According to one set of embodiments, a transducer is provided togenerate a standing wave field of vibrations. However, the sensitivestructure is disposed outside of the standing wave. (US 20130312787).For example, a transducer may be provided to launch standing waves intoa bilateral structure, with the heat source provided along an orthogonalaxis wherein vibrations from each side of the transducer bypass ordestructively interfere at the heat source. The standing wave isintended to cause movement of the fine feature elements of the heatsink,to dislodge debris.

The heatsink may also be cleansed by controlling factors such aspressure, temperature, nature of cleaning fluid (US 20050003737).

Other examples of situations, in which a heat sink has impairedefficiency: (a) pin fins have a lot of surface area, but the pins are soclose together that air has a hard time flowing through them; (b)aligning a heat sink so that the fins are not in the direction of flow;(c) aligning the fins horizontally for a natural convection heat sink.Whilst a heat sink is stationary and there are no centrifugal forces andartificial gravity, air that is warmer than the ambient temperaturealways flows upward, given essentially-still-air surroundings; this isconvective cooling.

The most common heat sink material is aluminum. Chemically pure aluminumis not used in the manufacture of heat sinks, but rather aluminumalloys. Aluminum alloy 1050A has one of the higher thermal conductivityvalues at 229 W/m·K. However, it is not recommended for machining, sinceit is a relatively soft material. Aluminum alloys 6061 and 6063 are themore commonly used aluminum alloys, with thermal conductivity values of166 and 201 W/m·K, respectively. The aforementioned values are dependenton the temper of the alloy.

Copper is also used since it has around twice the conductivity ofaluminum, but is three times as heavy as aluminum. Copper is also aroundfour to six times more expensive than aluminum, but this is marketdependent. Aluminum has the added advantage that it is able to beextruded, while copper cannot. Copper heat sinks are machined andskived. Another method of manufacture is to solder the fins into theheat sink base.

Another heat sink material that can be used is diamond. With a thermalconductivity value of 2000 W/m·K, it exceeds that of copper by a factorof five. In contrast to metals, where heat is conducted by delocalizedelectrons, lattice vibrations are responsible for diamond's very highthermal conductivity. For thermal management applications, theoutstanding thermal conductivity and diffusivity of diamond areessential. CVD diamond may be used as a sub-mount for high-powerintegrated circuits and laser diodes.

Composite materials also can be used. Examples are a copper-tungstenpseudoalloy, AlSiC (silicon carbide in aluminum matrix), Dymalloy(diamond in copper-silver alloy matrix), and E-Material (beryllium oxidein beryllium matrix). Such materials are often used as substrates forchips, as their thermal expansion coefficient can be matched to ceramicsand semiconductors.

Fin efficiency is one of the parameters which make a higher thermalconductivity material important. A fin of a heat sink may be consideredto be a flat plate with heat flowing in one end and being dissipatedinto the surrounding fluid as it travels to the other. As heat flowsthrough the fin, the combination of the thermal resistance of the heatsink impeding the flow and the heat lost due to convection, thetemperature of the fin and, therefore, the heat transfer to the fluid,will decrease from the base to the end of the fin. This factor is calledthe fin efficiency and is defined as the actual heat transferred by thefin, divided by the heat transfer were the fin to be isothermal(hypothetically the fin having infinite thermal conductivity). Equations5 and 6 are applicable for straight fins.

$\begin{matrix}{{\eta_{f} = \frac{\tanh \left( {mL_{c}} \right)}{mL_{c}}},} & (5) \\{{{m\; L_{c}} = \sqrt{\frac{2h_{f}}{kt_{f}}L_{f}}},} & (6)\end{matrix}$

Where h_(f) is the convection coefficient of the fin (Air: 10 to 100W/(m²·K), Water: 500 to 10,000 W/(m²·K)); k is the thermal conductivityof the fin material (Aluminum: 120 to 240 W/(m²·K)); L_(f) is the finheight (m); and t_(f) is the fin thickness (m).

Another parameter that concerns the thermal conductivity of the heatsink material is spreading resistance. Spreading resistance occurs whenthermal energy is transferred from a small area to a larger area in asubstance with finite thermal conductivity. In a heat sink, this meansthat heat does not distribute uniformly through the heat sink base. Thespreading resistance phenomenon is shown by how the heat travels fromthe heat source location and causes a large temperature gradient betweenthe heat source and the edges of the heat sink. This means that somefins are at a lower temperature than if the heat source were uniformacross the base of the heat sink. This non-uniformity increases the heatsink's effective thermal resistance.

A pin fin heat sink is a heat sink that has pins that extend from itsbase. The pins can be, for example, cylindrical, elliptical orsquare/geometric polygonal. A second type of heat sink fin arrangementis the straight fin. These run the entire length of the heat sink. Avariation on the straight fin heat sink is a cross cut heat sink. Astraight fin heat sink is cut at regular intervals but at a coarserpitch than a pin fin type.

In general, the more surface area a heat sink has, the better it works.However, this is not always true. The concept of a pin fin heat sink isto try to pack as much surface area into a given volume as possible. Aswell, it works well in any orientation. Kordyban has compared theperformance of a pin fin and a straight fin heat sink of similardimensions. Although the pin fin has 194 cm² surface area while thestraight fin has 58 cm², the temperature difference between the heatsink base and the ambient air for the pin fin is 50° C. For the straightfin it was 44° C. or 6° C. better than the pin fin. Pin fin heat sinkperformance is significantly better than straight fins when used intheir intended application where the fluid flows axially along the pinsrather than only tangentially across the pins. See, Kordyban, T., Hotair rises and heat sinks—Everything you know about cooling electronicsis wrong, ASME Press, NY 1998.

Another configuration is the flared fin heat sink; its fins are notparallel to each other, but rather diverge with increasing distance fromthe base. Flaring the fins decreases flow resistance and makes more airgo through the heat sink fin channel; otherwise, more air would bypassthe fins. Slanting them keeps the overall dimensions the same, butoffers longer fins. Forghan, et al. have published data on testsconducted on pin fin, straight fin and flared fin heat sinks. See,Forghan, F., Goldthwaite, D., Ulinski, M., Metghalchi, M., Experimentaland Theoretical Investigation of Thermal Performance of Heat Sinks,ISME, May. 2001. They found that for low approach air velocity,typically around 1 m/s, the thermal performance is at least 20% betterthan straight fin heat sinks. Lasance and Eggink also found that for thebypass configurations that they tested, the flared heat sink performedbetter than the other heat sinks tested. See, Lasance, C. J. M andEggink, H. J., A Method to Rank Heat Sinks in Practice: The Heat SinkPerformance Tester, 21st IEEE SEMI-THERM Symposium 2001.

The heat transfer from the heatsink is mediated by two effects:conduction via the coolant, and thermal radiation. The surface of theheatsink influences its emissivity; shiny metal absorbs and radiatesonly a small amount of heat, while matte black is a good radiator. Incoolant-mediated heat transfer, the contribution of radiation isgenerally small. A layer of coating on the heatsink can then becounterproductive, as its thermal resistance can impair heat flow fromthe fins to the coolant. Finned heatsinks with convective or forced flowwill not benefit significantly from being colored. In situations withsignificant contribution of radiative cooling, e.g. in case of a flatnon-finned panel acting as a heatsink with low airflow, the heatsinksurface finish can play an important role. Matte-black surfaces willradiate much more efficiently than shiny bare metal. The importance ofradiative vs. coolant-mediated heat transfer increases in situationswith low ambient air pressure (e.g. high-altitude operations) or invacuum (e.g. satellites in space). See, Fourier, J. B., 1822, Theorieanalytique de la chaleur, Paris; Freeman, A., 1955, translation, DoverPublications, Inc, NY; Kordyban, T., 1998, Hot air rises and heatsinks—Everything you know about cooling electronics is wrong, ASMEPress, NY; Anon, Unknown, “Heat sink selection”, Mechanical engineeringdepartment, San Jose State University [27 Jan. 2010];www.engr.sjsu.edu/ndejong/ME%20146%20files/Heat%20Sink.ppt; Sergent, J.and Krum, A., 1998, Thermal management handbook for electronicassemblies, First Edition, McGraw-Hill; Incropera, F. P. and DeWitt, D.P., 1985, Introduction to heat transfer, John Wiley and sons, NY;Forghan, F., Goldthwaite, D., Ulinski, M., Metghalchi, M., 2001,Experimental and Theoretical Investigation of Thermal Performance ofHeat Sinks, ISME May; Lasance, C. J. M and Eggink, H. J., 2001, A Methodto Rank Heat Sinks in Practice: The Heat Sink Performance Tester, 21stIEEE SEMI-THERM Symposium; ludens.cl/Electron/Thermal.html; Lienard, J.H., IV & V, 2004, A Heat Transfer Textbook, Third edition, MIT;Saint-Gobain, 2004, “Thermal management solutions for electronicequipment” 22 Jul. 2008www.fff.saint-gobain.com/Media/Documents/S0000000000000001036/ThermaCool%20Brochure.pdf;Jeggels, Y. U., Dobson, R. T., Jeggels, D. H., Comparison of the coolingperformance between heat pipe and aluminium conductors for electronicequipment enclosures, Proceedings of the 14th International Heat PipeConference, Florianópolis, Brazil, 2007; Prstic, S., Iyengar, M., andBar-Cohen, A., 2000, Bypass effect in high performance heat sinks,Proceedings of the International Thermal Science Seminar Bled, Slovenia,June 11-14; Mills, A. F., 1999, Heat transfer, Second edition, PrenticeHall; Potter, C. M. and Wiggert, D. C., 2002, Mechanics of fluid, ThirdEdition, Brooks/Cole; White, F. M., 1999, Fluid mechanics, Fourthedition, McGraw-Hill International; Azar, A, et al., 2009, “Heat sinktesting methods and common oversights”, Qpedia Thermal E-Magazine,January 2009 Issue; www.qats.com/cpanel/UploadedPdf/January20092.pdf.

Several structurally complex heatsink designs are discussed in Hernon,US 2009/032104.

The relationship between friction and convention in heatsinks isdiscussed in Frigus Primore, “A Method for Comparing Heat Sinks Based onReynolds Analogy,” available atakemalhammar.fr/downloads/Reynolds_analogy_heatsinks.PDF. This articlenotes that for, plates, parallel plates, and cylinders to be cooled, itis necessary for the velocity of the surrounding fluid to be low inorder to minimize mechanical power losses. However, larger surface flowvelocities will increase the heat transfer efficiency, especially wherethe flow near the surface is turbulent, and substantially disrupts astagnant surface boundary layer. Primore also discusses heatsink finshapes and notes that no fin shape offers any heat dissipation or weightadvantage compared with planar fins, and that straight fins minimizepressure losses while maximizing heat flow. Therefore, the art generallyteaches that generally flat and planar surfaces are appropriate for mostheatsinks.

Frigus Primore, “Natural Convection and Inclined Parallel Plates,”www.engineeringclicks.com/natural-convection-and-inclined-parallel-plates/,discusses the use of natural convection (i.e., convection due to thethermal expansion of a gas surrounding a solid heatsink in normaloperating conditions) to cool electronics. One of the design goals ofvarious heatsinks is to increase the rate of natural convection. Primoresuggests using parallel plates to attain this result. Once again,Primore notes that parallel plate heat sinks are the most efficient andattempts to define the optimal spacing and angle (relative to thedirection of the fluid flow) of the heat sinks according to theequations in FIG. 1:

$\begin{matrix}{{Optimum}\mspace{14mu} {Plate}\mspace{14mu} {Spacing}} & \; \\{{S_{opt} = {{{k_{s}\left( \frac{L}{dT} \right)}^{0.25} \cdot \cos}\; (\gamma)^{- 0.25}}}{\gamma_{opt} = {{{{atan}\left( \frac{1H}{3W} \right)}\mspace{14mu} \frac{H}{W}} < \sqrt{3}}}{{{\gamma_{opt}\frac{\_\pi}{4}} - {{0.5}08\left( \frac{H}{W} \right)^{- 1.237}\mspace{14mu} \frac{H}{W}}} > \sqrt{3}}} & (1) \\{{Total}\mspace{14mu} {heat}\mspace{14mu} {Dissipation}} & \; \\{{\overset{.}{Q} = {k_{v} \cdot k_{\gamma} \cdot A_{c} \cdot H^{0.5} \cdot {dT}^{1.5}}}{k_{\gamma} = {{\sqrt{1 + {\frac{1}{9}\left( \frac{H}{W} \right)^{2}}}\mspace{14mu} \frac{H}{W}} < \sqrt{3}}}{k_{\gamma} = {{{0{{.307} \cdot \left( \frac{H}{W} \right)^{- {0.5}}}} + {0{{.696} \cdot \left( \frac{H}{W} \right)^{- 0.5}}\mspace{14mu} \frac{H}{W}}} > \sqrt{3}}}} & (2) \\{{Applied}\mspace{14mu} {Equation}} & \; \\{{\overset{.}{Q} = {\eta_{v} \cdot k_{v} \cdot k_{\gamma} \cdot A_{c} \cdot H^{0.5} \cdot {dT}_{ref}^{1.5}}}{{dT} = {{Temperature}\mspace{14mu} {difference}\mspace{11mu} (K)}}{A_{c} = {W \cdot D}}{\eta_{v} = {{Volumetric}\mspace{14mu} {{efficiency}\;\lbrack--\rbrack}}}{\overset{.}{Q} = {{Heat}\mspace{14mu} {{dissipation}\mspace{11mu}\lbrack W\rbrack}}}} & (3)\end{matrix}$

In another article titled “Natural Convection and Chimneys,” availableat akemalhammar.fr/articels2/parallel_pl_Inc.html, discusses the use ofparallel plates in chimney heat sinks. One purpose of this type ofdesign is to combine more efficient natural convection with a chimney.Primore notes that the design suffers if there is laminar flow (whichcreates a re-circulation region in the fluid outlet, thereby completelyeliminating the benefit of the chimney) but benefits if there isturbulent flow which allows heat to travel from the parallel plates intothe chimney and surrounding fluid.

Batten, Paul, et al. “Sub-Grid Turbulence Modeling for Unsteady Flowwith Acoustic Resonance,” available atwww.researchgate.net/publication/269068673_Sub-grid_turbulence_modeling_for_unsteady_flow_with_acoustic_resonance,discuss that when a fluid is flowing around an obstacle, localizedgeometric features, such as concave regions or cavities, create pocketsof separated flow which can generate self-sustaining oscillations andacoustic resonance. The concave regions or cavities serve tosubstantially reduce narrow band acoustic resonance as compared to flatsurfaces. This is beneficial to a heat sink in a turbulent flowenvironment because it allows for the reduction of oscillations andacoustic resonance, and therefore for an increase in the energyavailable for heat transfer.

Liu, S., et al., “Heat Transfer and Pressure Drop in FractalMicrochannel Heat Sink for Cooling of Electronic Chips,” 44 Heat MassTransfer 221 (2007), discuss a heatsink with a “fractal-like branchingflow network.” Liu's heatsink includes channels through which fluidswould flow in order to exchange heat with the heatsink.

Y. J. Lee, “Enhanced Microchannel Heat Sinks Using Oblique Fins,” IPACK2009-89059, similarly discusses a heat sink comprising a “fractal-shapedmicrochannel based on the fractal pattern of mammalian circulatory andrespiratory system.” Lee's idea, similar to that of Liu, is that therewould be channels inside the heatsink through which a fluid could flowto exchange heat with the heatsink. The stated improvement in Lee'sheatsink is (1) the disruption of the thermal boundary layerdevelopment; and (2) the generation of secondary flows.

Pence, D. V., 2002, “Reduced Pumping Power and Wall Temperature inMicrochannel Heat Sinks with Fractal-like Branching Channel Networks”,Microscale Thermophys. Eng. 5, pp. 293-311, mentions heatsinks that havefractal-like channels allowing fluid to enter into the heat sink. Thedescribed advantage of Pence's structure is increased exposure of theheat sink to the fluid and lower pressure drops of the fluid while inthe heatsink.

In general, a properly designed heatsink system will take advantage ofthermally induced convection or forced air (e.g., a fan). In general, aturbulent flow near the surface of the heatsink disturbs a stagnantsurface layer, and improves performance. In many cases, the heatsinkoperates in a non-ideal environment subject to dust or oil; therefore,the heatsink design must accommodate the typical operating conditions,in addition to the as-manufactured state.

Therefore, two factors appear to conflict in optimizing theconfiguration of an external heat exchange surface: the surfaceconfiguration designed to disturb laminar flow patterns, createturbulence, and enhance convective heat transfer, and the desire toefficiently flow large volumes of heat transfer fluid (e.g., air), overthe surfaces, which is enhanced by laminar (smooth) flow. Even inpassive dissipative device, convective flow may be a significant factor,and reducing air flow volume and velocity by increasing the effectiveimpedance can be counterproductive. On the other hand, in some cases,the amount of energy necessary to move the air is dwarfed by the problemto be solved. In many computing systems, the processors are thermallyconstrained, that is, the functioning of the processor is limited by theability to shed heat. In such cases, innovative ways to improve theefficiency of heat transfer may yield significant benefit, even if insome regimes of operation, they impose certain inefficiencies.

Prior art heatsink designs have traditionally concentrated on geometrythat is Euclidian, involving structures such as the pin fins, straightfins, and flares discussed above.

N J Ryan, D A Stone, “Application of the FD-TD method to modelling theelectromagnetic radiation from heatsinks”, IEEE International Conferenceon Electromagnetic Compatibility, 1997. 10th (1-3 Sep. 1997), pp:119-124, discloses a fractal antenna which also serves as a heatsink ina radio frequency transmitter.

Lance Covert, Jenshan Lin, Dan Janning, Thomas Dalrymple, “5.8 GHzorientation-specific extruded-fin heatsink antennas for 3D RF systemintegration”, 23 Apr. 2008 DOI: 10.1002/mop.23478, Microwave and OpticalTechnology Letters Volume 50, Issue 7, pages 1826-1831, July 2008 alsoprovide a heatsink which can be used as an antenna.

See, U.S. 20140098542; 20130309778; 20130286666; 20130155687;20130042893; 20120174650; 20120031272; 20110280019; 20110226460;20090045967; 20090021270; 20070041159; 20060072289; U.S. Pat. Nos.8,784,540; 8,764,243; 8,602,599; 8,539,840; 8,506,674; 8,491,683;7,696,890; 7,113,402; and 5,856,836.

SUMMARY OF THE INVENTION

In a preferred embodiment, a heatsink employed according to the presenttechnology provides a branched 3D network of elements, which have a highsurface area and, especially, near the terminal branches, havemicrostructured surfaces. As a result, dust may accumulate on thesurfaces, resulting in decreased surface area as the dust obscures themicrostructuring, increased thermal resistance, and deoptimized air flowpatterns. Therefore, various technologies may be provided to reduce oreliminate the dust deposition on the surfaces than may occur inreal-world environments:

-   -   vacuum environment;    -   filtering of the incoming air which circulates around the        heatsink, e.g., using a HEPA filter;    -   clean liquid heat transfer medium;    -   air jet(s) which operate continuously or periodically to        dislodge dust particles;    -   inducing electrostatic charge on the dust particles to repel        them from the surface of the heatsink;    -   high voltage electric fields with or without electric discharge        to cause the dust particles to move in response to the fields;    -   pyrolizing the dust (or binding factors within the dust that        cause sticking to the surfaces), such as by intermittent IR        laser emissions, resistive heating of the terminal branches of        the heatsink, radiative heating, combustion, or other processes;    -   narrowband vibration over a range of frequencies representing        resonances in the branches of the heatsink;    -   impulse vibration (e.g., from a piezoelectric transducer);    -   use of shape memory alloys, and causing a transition through the        Curie temperature to induce significant shape change, resulting        in surface stresses to dislodge dust.

In various cases, the physical effect sought to be employed to preventdust accumulation or to dislodge the dust may impair or risk impairingthe device being protected by the heatsink. For example, pyrolysis ofthe dust or its binder requires high temperatures, which would normallydiffuse back to the device being protected. However, according to thepresent technology, this can be prevented by focusing the effect on theterminal branches of the heatsink. For example, a catalytic coating onthe tips of the heatsink branches would permit a sub-explosive mixtureof a combustible gas to combust at the catalyst, while air flows onother portions of the heatsink could protect the device fromoverheating. Similarly, use of an infrared (or other wavelength) pulselaser would cause two distinct effects to reduce dust accumulation:first, the laser would heat the immediate region of the heatsinkirradiated by the laser, and second, the rapid thermal expansion of theheated material would generate a pressure shock wave that would tend tofracture the aggregated dust material; the amount of heat used need notbe significant with respect to the heat load capacity of the heatsink.Therefore, heat-based technologies may acceptably be employed to removedust.

A current flowing through the branched heatsink would cause heating,with higher temperatures at the narrow terminal branches. The voltagesassociated with high currents flowing through a metallic heatsink arelow, and would generally not cause damage to electronics unless flowingthrough a forward-biased junction. Therefore, if the heating wasperformed as periodic pulses, the tips of the branched network wouldheat, and that heating would tend to reduce binding of the dust andcause a thermal expansion that would cause a strain in the dust layerthat would reduce adhesion. Therefore, the dust layer would bedisrupted. Similarly, a current passing through a shape memory allowsuch as Nitinol, would cause mechanical stresses and strains that coulddisrupt a dust layer, without requiring high temperature excursionsbeyond the Curie temperature of the material.

The use of vibration, wither continuous or pulsed, may cause damage tothe device protected by the heatsink. One way to protect the device isthrough a liquid or gel-like layer that does not support propagation ofshear waves, and exciting the vibrations in a shear mode. As such, theliquid or gel interface, which may be a heat transfer paste, may isolatethe device to be protected from the vibration. In some cases, an activevibration suppression technology may be employed, such as apiezoelectric transducer that emits a feedback-controlled vibration toreduce the vibration experienced by the device to be protected.

Vibration is particularly interesting as a physical process to dislodgedust and debris and to maintain cleanliness of the heat exchangesurfaces. The process imposes three constraints: first, vibration itselfcreates heat, which may or may not be a significant factor; second, manysystems which generate heat may themselves be sensitive to vibration,for example bond wires of integrated circuits; and third, significantvibration may lead to fracture failure of the heatsink, bearing in mindthat the vibration should have sufficient amplitude to generate inertiain the particulates at the surface to overcome the adhesive forces, andthe movement may also create turbulence in the surrounding heat exchangemedia. In order to overcome these issues, a vibration isolator may beprovided between the heat source and the source of vibration, which maybe, for example, a piezoelectric element or electromagnetic element. Thevibration isolator may be, for example, a plastic thermal transfermedium (e.g., a paste), a non-shear transmissive solid, such as a wirebundle (e.g., thin copper wires), an active vibration suppressioninterface, or the like. Further, the branching or fractalcharacteristics of the heatsink may be exploited to selectively transmitthe vibrations distally from the source, by selecting the frequency orfrequency range, vibration mode, etc., to generate significant movementof the distal branches of the heatsink, without unduly stressing theheat source. In another embodiment, the heatsink is “separable” from theheat source over a short period, and during that period, a large impulseis launched in the heatsink, to dislodge dust; the heatsink isthereafter reconnected to the source. In another embodiment, theheatsink is supported on an anisotropic mount (e.g., piston incylinder), that provides good heat transfer, but does not supporttransmission of vibrations along at least one axis, which is then usedas the axis of excitation for dust removal.

Electrostatic charge may cause damage to semiconductors. In order forelectrostatic repulsion to be effective, the dust particles should becharged to a net charge of the same polarity as the heatsink, with anoppositely charged collector in the air flow path to divert the dust.The dust may be charged with a radioisotope, typically a low energyalpha or beta particle emitter, or through induced charge by chargedplates, screen, or electron emitting surface. The voltages willtypically be in the hundreds or low thousands of Volts, and in anenvironment that maintains such potential differences, discharge eventsmay be common. If the device being protected or other nearby devices arestatic or static discharge sensitive, the result may be damage to thesensitive components. One way to reduce this issue is to dielectricallyisolate the protected device from the electrically charged heatsink. Forexample, a high thermal conductivity layer, such as a diamond-likelayer, may be provided as an electrical conduction barrier, with thesensitive device-side electrically grounded. This configuration mightpermit (or in some cases, encourage) dust deposition on the groundedportion of the heatsink and sensitive device; however, the structure ofthe branched network is such that accumulation of dust at the root ofthe branching network does not substantially impair heatsink operation.

In a vacuum environment, no dust is present, but convective heattransfer is prevented. Nevertheless, heat transfer through radiation maybe sufficient, as fractal structures are known to be extremely efficientantennas for transmitting electromagnetic radiation. If the air passingover the heatsink is filtered with a HEPA filter, the presence of dustis significantly reduced, but the air-movement efficiency is alsoimpaired. Air jets (typically with filtered air) may be aimed atportions of the heatsink and used to blow away accumulated dust. Thesehave low efficiency with respect to the air in the jet being used as aheat transfer medium, but are efficient in removing dust. In some cases,an air jet can consolidate and densify loose dust, and therefore suchjets should operate frequently, before significant dust accumulation canoccur.

In a vacuum or radiative embodiment, the design of the exterior surfacemay be optimized to maximize emission (generally by maximizing hotsurface area), and minimize recapture of radiated heat, among otherconstraints. This can generally be met by texturing the surface andavoiding hot parallel surfaces and especially by inclining surfaces withrespect to each other. In three dimensions, the optimized radiantheatsink may be fractal, since these can be optimized to have highsurface area, and obtuse relative angles. Likewise, more distal portionsof the heatsink from the heat source may have more reflective surfacesthan proximal portions, which may have greater emissivity. When suchdevices are subject to convective cooling as well, the recapture may beless of a factor in the overall efficiency, but should not be ignored,especially at the high temperature regions of the heatsink. In theconvective case, the fluid may have entrained particles or dust, andthis dust may be captured by or adhere to surfaces of the heatsink,reducing its efficiency by changing the shape and surface emissivitycharacteristics, changing the heat diffusion characteristics within thesolid phases, and impairing convection.

A time-varying flow of a fluid, e.g., a heat exchange media, can beprovided over the heatsink. This achieves a number of advantages. First,while a high flow rate peak may be an inefficient use of energy in termsof running the fan or pump, the high flow rate may assist in dislodgingdust by inertia and turbulence. Second, the changes in flow rate willtend to create time-varying tensor flow patterns that increase theprobability that the dust or debris will be at least temporarilydislodged from the surface and available for advective flow in the fluidmedium. Third, the time varying flow, especially peak rates, can disruptsurface fluid boundary layers, increasing the advective component of theconvective transfer process.

In some cases, non-adhesive particles may be entrained in the stream, toassist in removing surface debris. That is, while normally dust andparticulates are sought to be avoided near the heatsink, byintentionally entraining specific particles, dust removal may befacilitated. For example, relatively dense particles entrained in afluid flow can impact the surfaces of the heatsink, and as a resultdislodge adherent dust or particles. While often a minor effect, theparticles can themselves participate in conductive and advective heattransfer. Further, the heterogeneous fluid with particles can enhanceturbulent flow patterns around the surfaces of the heatsink, enhancingheat flow from the surfaces. The heatsink system is typically a closedsystem, and therefore the entrained particles may then be recollected,filtered (to remove the undesired dust), and reused. In one embodiment,the particles are magnetic, and can therefore be collected magnetically,relaxing the need for a sealed system. Similarly, electrostatic particlecollection technology may be employed. With respect to magneticparticles, the heatsink itself may be periodically magnetized, for causethe cool magnetic particles to stick, and thereafter demagnetized,permitting the heated magnetic particles to become free and entrained inthe surrounding fluid, thus enhancing the advective heat transferprocess. Similarly, in a vacuum or low pressure environment, transientcontact of the particles (magnetic or otherwise) with the heat transfersurfaces may facilitate advective heat transfer as an adjunct toradiative heat transfer, and thus the particles need not be entrained ina fluid.

The result of the fluid flow process, especially under dynamicallychanging conditions, can be complex. For example, the flow can causeturbulent flow around the heat exchange elements, which induce complexpressure differentials, vibrations, and inertial flow patterns.Dynamically changing the flow rate or flow pattern can help distributethe turbulent dynamics over various regions of the heatsink surface.Thus, the entire surface of the heatsink need not be subject tocontinual high fluid flow rates, and only a small portion of the surfaceat any given time might be subject to a “jet” of fluid flow, thusreducing the energy disadvantage. Meanwhile, the jet may bestrategically focused on portions of the heatsink intended to achieveparticular effects. When the jet (or more generally, high flow ratestream) is focused or directed at the hot portion of the heatsink,higher convective heat transfer will occur. However, discontinuous highflow rates may be advantageous, since a reduced fluid flow on a regionwill tend to cause a diffusive heat transfer to the heat transfermaterial underlying the cooled surface, and thus lead to higherefficiency heat transfer when the jet or stream returns. Meanwhile, thejet or stream can be directed to other portions of the heatsink. This,in turn, causes dynamic temperature gradients within the heatsink, whichcan be controlled to causes pulsatile heating at the periphery of theheatsink, especially in a branched network. Thus, for example, in afractal branched heatsink, the stream of fluid can be controlled topermit various regions of the heatsink to undergo heating and coolingcycles, such that the hot spots on the heatsink are dynamicallycontrolled to be selectively cooled. While a model of the process may beemployed, sensors, such as thermal sensors, thermal cameras, passiveinfrared sensors, optical camera with thermally responsive coating onthe heatsink, or the like, may be used to monitor surface temperaturesof the heatsink, and adaptively supply coolant as appropriate. Sensorsmay also be used to detect surface contamination of the heatsink, and aneed for removal of the contamination, which may be by fluid jet,entrained particles, mechanical debris removal, etc.

The fluid flow over the heatsink surface can also cause acousticresonance, which in the case of a heatsink having a fractal geometry,can be, in the aggregate, a broadband resonance. The flow can bepulsatile, with pulses causing inertial transfer of energy to the debrison the surface, resulting in separation from the underlying heatexchange surface. The flow can also cause stress and strain on thedebris coating on the surface, causing separation along the surfaceplane. In some, the time varying flow can effectively remove theaccumulated surface debris. A static flow in some cases could alsoreduce accumulation, but it is noted that the static flow is presumed tobe associated with the accumulation conditions, and maintenance ofsufficient continuous flow conditions to remove accumulation may consumeexcess energy, noise, and abrasion of the heat exchange surfaces.

Liquid heatsinks typically provide for a flow of liquid within a tube orchannel or confined space. (In some cases, a spray of a non-volatilefluid over an open heat transfer surface is provided, similar to amachining process). As a result, a relatively large body of heattransfer material is provided with channels provided therein. In such adesign, the cross-section area of the channels is relatively constant inthe aggregate as the fluid travels through the branched channels. As aresult, the linear velocity of the fluid flow will be constant. However,when one considers the logistics of a typical design, the flow channelsare either planar or the design is radially symmetric.

In a planar configuration, a base of the heatsink interfaces with theheat source, and the fluid flows through the structure above the heatsource to withdraw heat. See, Escher, W., B. Michel, and D. Poulikakos“Efficiency of optimized bifurcating tree-like and parallel microchannelnetworks in the cooling of electronics.” International Journal of Heatand Mass Transfer 52.5 (2009): 1421-1430; Wang et al., “Flow and ThermalCharacteristics of Offset Branching Network,” 12 Aug. 2009,International Journal of Thermal Science, Vol. 49, Pages 272-280;Yongping, Chen, et al. “Characteristics of Heat and Fluid Flow inFractal Tree-like Channel Heat Sink [J].” Acta Aeronautica EtAstronautica Sinica 3 (2010): 008; Xu, Peng, et al. “Thermalcharacteristics of tree-shaped microchannel nets with/without loops.”International Journal of Thermal Sciences 48.11 (2009): 2139-2147; Liu,Shutian, Yongcun Zhang, and Peng Liu. “Heat transfer and pressure dropin fractal microchannel heat sink for cooling of electronic chips.” Heatand Mass Transfer 44.2 (2007): 221-227; Alharbi, Ali Y., Deborah V.Pence, and Rebecca N. Cullion. “Thermal characteristics of microscalefractal-like branching channels.” Journal of Heat Transfer 126.5 (2004):744-752; Hong, F. J., et al. “Conjugate heat transfer in fractal-shapedmicrochannel network heat sink for integrated microelectronic coolingapplication.” International Journal of Heat and Mass Transfer 50.25(2007): 4986-4998; Lee, Yong-Jiun, Poh-Seng Lee, and Siaw-Kiang Chou.““Enhanced microchannel heat sinks using oblique fins.” ASME 2009InterPACK Conference collocated with the ASME 2009 Summer Heat TransferConference and the ASME 2009 3rd International Conference on EnergySustainability, American Society of Mechanical Engineers, 2009; Senn, S.M., and D. Poulikakos. “Laminar mixing, heat transfer and pressure dropin tree-like microchannel nets and their application for thermalmanagement in polymer electrolyte fuel cells.” Journal of Power Sources130.1 (2004): 178-191; Xiangqi, Wang. “New approaches tomicro-electronic component cooling.” PhD diss., 2007 (NationalUniversity of Singapore); U.S. Pat. No. 6,688,381; 2008037927; U.S. Pat.Nos. 6,333,852; 7,256,751. The temperature gradient within the heatsinkhaving a planar flow plane would generally be decreasing with distanceaway from the interface, with the bulk material in and near the fluidflow plane largely isothermal.

In a radially symmetric arrangement, typically a constant cross sectionbranched solid heatsink (e.g., extruded), see e.g., U.S. Pat. No.4,715,438; 20080080137, 20090050293; U.S. Pat. Nos. 8,295,046;2,535,721, may be placed within a shell or confinement, and a containedfluid permitted to contact the exposed surfaces. In this case, the fluidpath is not highly constrained, and the operating temperature may beunstable, for example due to nearly adiabatic movement of fluid massesas a result of density and viscosity differences of the heated fluid. Anextruded heatsink is generally a suboptimal shape, since the more distalportions of the structure a constant higher surface by lower thermalgradient. Indeed, due to possible adiabatic movement of hot fluid, insome cases the fluid can heat portions of the heatsink.

A “structurally complex” heatsink is provided in US 20090321045, butwithout branching networks and without optimized regional heterogeneity.

In a closed, vacuum or filtered system, typically no accumulation ofdust, debris or precipitate on the heat exchanger surface occurs.

The techniques discussed above may be classified in five schemes:prevention of deposition; mechanical removal of deposition; thermaldegradation of typically organic material; shock waves or vibrations todisrupt surface layer debris; and dynamic configuration.

Most heatsinks are designed using a linear or exponential relationshipof the heat transfer and dissipating elements. A known geometry whichhas not generally been employed is fractal geometry. Some fractals arerandom fractals, which are also termed chaotic or Brownian fractals andinclude random noise components. In deterministic fractal geometry, aself-similar structure results from the repetition of a design or motif(or “generator”) using a recursive algorithm, on a series of differentsize scales. As a result, certain types of fractal images or structuresappear to have self-similarity over a broad range of scales. On theother hand, no two ranges within the design are identical.

A fractal is defined as “a rough or fragmented geometric shape that canbe split into parts, each of which is (at least approximately) areduced-size copy of the whole.” Mandelbrot, B. B. (1982). That is,there is a recursive algorithm which describes the structure. TheFractal Geometry of Nature. W.H. Freeman and Company. ISBN0-7167-1186-9. This property is termed “self-similarity.” For a moredetailed discussion of fractals, see the Wikipedia article thereon aten.wikipedia.org/wiki/Fractal. Exemplary images of well-known fractaldesigns can also be viewed on the Wikipedia page. Due to the fact thatfractals involve largely self-repeating patterns, each of which servesto increase the surface area in three-dimensional fractals (perimeter intwo-dimensional fractals), three dimensional fractals in theory arecharacterized by infinite surface area (and two-dimensional fractals arecharacterized by infinite perimeter). In practical implementations, thescale of the smallest features, which remain true to the generatingalgorithm, may be 3-25 iterations of the algorithm. Less than threerecursions, and the fractal nature is not apparent, while presentmanufacturing technologies limit the manufacture of objects with a largerange of feature scales.

An “approximately” fractal structure is one that, while a true fractalis a result of infinite number of iterations leading sometimes toinfinite length of the border (such as Koch snowflake), in reality, anymanufactured fractal will be a result of finite number of iterations inthe fractal algorithm: 2 or 3, but rarely more than 5 or 6. Theapproximate fractal design may display various symmetries, and typicallyhas a branched architecture with a tapering cross section from the heatsource to the periphery.

Fractal theory is related to chaos theory. See,en.wikipedia.org/wiki/Chaos_theory. See, Sui, Y., Teo, C. J., Lee, P.S., Chew, Y. T., & Shu, C. (2010). Fluid flow and heat transfer in wavymicrochannels. International Journal of Heat and Mass Transfer, 53(13),2760-2772; Garibaldi, Dott Ing Pietro. Single-phase natural circulationloops: effects of geometry and heat sink temperature on dynamic behaviorand stability. Diss. Ph. D. Thesis, 2008; Fichera, A., and A. Pagano.“Modelling and control of rectangular natural circulation loops.”International journal of heat and mass transfer 46.13 (2003): 2425-2444;Fichera, Alberto, et al. “A modeling strategy for rectangular thermalconvection loops.” World Congress. Vol. 15. No. 1. 2002; Crane, JacksonT. Radial parallel plate flow with mechanical agitation. Diss.Massachusetts Institute of Technology, 2013.

This fractal nature is useful in a heatsink because the rate at whichheat is transferred from a surface, either through convection or throughradiation, is typically related to, and increasing with, the surfacearea. Of course, due to limitations in the technology used to buildthese heatsinks, engineering compromise is expected. However a featureof an embodiment of the designs proposed herein is that vortices inducedby fluid flow over a heat transfer surface will be chaoticallydistributed over various elements of the surface, thus disrupting thestagnant surface boundary layer and increasing the effective surfacearea available for heat transfer, while avoiding acoustic resonancewhich may be apparent from a regular array of structures which producevortices and turbulence.

Further, a large physical surface area to volume ratio, which isgenerally useful in heatsink design, can still be obtained using thefractal model. In addition, fractal structures provide a plurality ofconcave regions or cavities, providing pockets of separated flow whichcan generate self-sustaining oscillations and acoustic resonance. Thesepockets serve to reduce the acoustic resonance in turbulent flowingfluid (as compared to flat or Euclidian surfaces), thus allowing formore effective heat transfer between the fractal structure and thesurrounding fluid, thereby making the fractal structure ideal for aheatsink.

U.S. Pat. No. 7,256,751 (Cohen), discusses fractal antennas. In thebackground of this patent, Cohen discusses Kraus' research, noting thatEuclidian antennas with low area (and therefore low perimeter) exhibitvery low radiation resistance and are thus inefficient. Cohen notes thatthe advantages of fractal antennas, over traditional antennas withEuclidian geometries, is that they can maintain the small area, whilehaving a larger perimeter, allowing for a higher radiation resistance.Also, Cohen's fractal antenna features non-harmonic resonancefrequencies, good bandwidth, high efficiency, and an acceptable standingwave ratio.

In the instant invention, this same wave theory may be applied tofractal heatsinks, especially with respect to the interaction of theheat transfer fluid with the heatsink. Thus, while the heat conductionwithin a solid heatsink is typically not modeled as a wave (thoughmodern thought applies phonon phenomena to graphene heat transport), thefluid surrounding the heating certainly is subject to wave phenomena,complex impedances, and indeed the chaotic nature of fluid eddies mayinteract with the chaotic surface configuration of the heatsink.

The efficiency of capturing electric waves in a fractal antenna,achieved by Cohen, in some cases can be translated into an efficiencytransferring heat out of an object to be cooled in a fractal heatsink asdescribed herein. See, Boris Yakobson, “Acoustic waves may coolmicroelectronics”, Nano Letters, ACS (2010). Some physics scholars havesuggested that heat can be modeled as a set of phonons. Convection andthermal radiation can therefore be modeled as the movement of phonons. Aphonon is a quasiparticle characterized by the quantization of the modesof lattice vibration of solid crystal structures. Any vibration by asingle phonon is in the normal mode of classical mechanics, meaning thatthe lattice oscillates in the same frequency. Any other arbitrarylattice vibration can be considered a superposition of these elementaryvibrations. Under the phonon model, heat travels in waves, with awavelength on the order of 1 μm. In most materials, the phonons areincoherent, and, therefore, on macroscopic scales, the wave nature ofheat transport is not apparent or exploitable.

The thermodynamic properties of a solid are directly related to itsphonon structure. The entire set of all possible phonons combine in whatis known as the phonon density of states which determines the heatcapacity of a crystal. At absolute zero temperature (0 Kelvin or −273Celsius), a crystal lattice lies in its ground state, and contains nophonons. A lattice at a non-zero temperature has an energy that is notconstant, but fluctuates randomly about some mean value. These energyfluctuations are caused by random lattice vibrations, which can beviewed as a gas-like structure of phonons or thermal phonons. However,unlike the atoms which make up an ordinary gas, thermal phonons can becreated and destroyed by random energy fluctuations. In the language ofstatistical mechanics this means that the chemical potential for addinga phonon is zero. For a more detailed description of phonon theory, seethe Wikipedia article thereon available at en.wikipedia.org/wiki/Phonon.

In certain materials, such as graphene, phonon transport phenomena areapparent at macroscopic levels, which make phonon impedance measurableand useful. Thus, if a graphene sheet were formed to resonate at aparticular phonon wavelength, the resonant energy would not be emitted.On the other hand, if the graphene sheet were configured using a fractalgeometry, the phonon impedance would be well controlled over a broadrange of wavelengths, with sharp resonances at none, leading to anefficient energy dissipation device.

One aspect of the technology therefore employs a thermally responsivetechnology, such as a memory metal or bimetal element actuator (whichmay be passive or active) (see en.wikipedia.org/wiki/Bimetallic strip;www.emsclad.com/fileadmin/Data/Divisions/EMS/Bimetal_Desingers_Guide.pdf),or other active or passive element, to change the configuration of theheatsink under various conditions. It is noted that in an automotiveradiator, a thermostat is provided to shunt flow around the radiatorwhen the engine is cool. This is distinguished herein, in variousalternate ways. For example, a variable geometry heatsink according tothe present technology may have an external surface exposed to anunconstrained heat transfer medium, such as air. See, Baurle, R. A., andD. R. Eklund. “Analysis of dual-mode hydrocarbon scramjet operation atMach 4-6.5.” Journal of Propulsion and Power 18.5 (2002): 990-1002;Cockrell Jr, Charles E. “Technology Roadmap for Dual-Mode ScramjetPropulsion to Support Space-Access Vision Vehicle Development.” (2002);Boudreau, Albert H. “Hypersonic air-breathing propulsion efforts in theair force research laboratory.” AIAA 3255.1 (2005): 10; Kay, Ira W., W.T. Peschke, and R. N. Guile. “Hydrocarbon-fueled scramjet combustorinvestigation.” Journal of Propulsion and Power 8.2 (1992): 507-512;Jackson, K., et al. “Calibration of a newly developed direct-connecthigh-enthalpy supersonic combustion research facility.” AIAA paper(1998): 98-1510; Donbar, J., et al. “Post-test analysis of flush-wallfuel injection experiments in a scramjet”, AIAA Paper 3197 (2001): 2001;Gruber, Mark, et al. “Newly developed direct-connect high-enthalpysupersonic combustion research facility.” Journal of Propulsion andPower 17.6 (2001): 1296-1304; Andrews, Earl H. “Scramjet development andtesting in the United States”, AIAA paper 1927 (2001): 2001; Palac,Donald T., Charles J. Trefny, and Joseph M. Roche, PerformanceEvaluation of the NASA GTX RBCC Flowpath, NASA, Glenn Research Center,2001; US 20030155110; 20040187861; 20050245659; 20090016019;20090321047; 20100089549; 20100236236, 20100252648; 20110174462;20120293952; 20140360699; U.S. Pat. Nos. 4,654,092; 4,931,626;5,371,753; 5,483,098; 5,548,481; 5,510,598; 6,128,188; 6,330,157;6,689,486; 7,080,989; 7,778,029; 8,228,671; 8,385,066; JP 03-070162; JP04-291750; JP 61-098565; JP 63-006915; WO 99/04429.

For example, in one embodiment, a thermodynamic model of the system,encompassing at least the heat source, the heat sink, the thermaltransfer medium, and a device to induce thermal transfer medium flow,determines, under each set of conditions, the optimal configuration. Forexample, at low loads, the heat sink may operate passively, withoutflows induced by an active device to induce flow in the thermal transfermedium. In such a case, radiative heat transfer may be important, aswell as thermally-induced convection. Under high loads, the activedevice to induce flow in the thermal transfer medium may induce maximumflows, and the heatsink configured for minimal turbulence with laminarflows where possible. In intermediate states, the system may assume aconfiguration which is optimized according to a cost function, which mayinvolve the effect of heat/temperature on the heat source, energyconsumed by the active device to induce flow in the thermal transfermedium, noise resulting from induced flow, etc. This allows efficientuse of an “oversized” heatsink, since the heatsink characteristics arevariably controlled. In these intermediate states of configuration,efficiency may be improved by allowing the heatsink to assume a variableconfiguration. Since the optimum heatsink configuration depends on,e.g., ambient temperature, humidity, atmospheric pressure, heat load,air flow rate, gravitational vector with respect to the heatsink, etc.,the model should explore the range of combinations of the device toinduce thermal transfer medium flow, the variable geometry, and to alesser extent, control over the heat source. An example of the later isthat for a given power dissipation, it may be more efficient to havethermal cycles reaching a maximum temperature than a constanttemperature. During the cycles, the geometry may change. Indeed, if thesystem is not in a static steady state, the geometry may optimallychange during or in anticipation of temperature changes. An example hereis that as the heat source produces a heat peak, the heat diffuses overtime through a solid heatsink material. There is a lag, and so thetemperature of the heat source is different that the temperature of theheatsink, and the heatsink itself has variations in temperature atdifferent positions. Typically, there is a single actuator whichcontrols the entire heatsink, though this is not a limitation, and theremay be multiple actuators to control different parts of the heatsinkindependently or semi-independently. The device to induce thermaltransfer medium flow may have a variable flow rate, and also may havemultiple independently controlled portions. However, as the heat beginsto peak, the device to induce thermal transfer medium flow may alsoincrease activity. This, in turn, can reduce the temperature of variousportions of the heatsink, depending on the relationship of the device toinduce thermal transfer medium flow and the variable geometry heatsink.Thus, the entire system may operate in a phased cyclic or dynamicmanner, with asynchronous maxima and minima of the various functions.

In practice, a heatsink may be provided for a microprocessor havingmultiple cores. Under low load, the device to induce thermal transfermedium flow may be off, or at a low flow rate. The heatsink in this caseoptimally has the greatest spread for radiative and passive convectivecooling. In case of a higher load, the processor itself may have theoption of distributing the load over multiple cores, and spatiallyspreading the heat dissipation, or concentrating the load in a singlecore which may get hot. Since temperature differentials increase heatflow, the concentrated heat source may selectively transfer heat tosub-portion of the heatsink, and thus that portion may be able toefficiently shed the heat under the passive or low energy cost state. Asthe load further increases, the processor as a whole typically becomesthermally limited, and as a result, the entire die or processor complexis treated as a unitary source, spreading heat to all elements of theheatsink. Initially, the temperature is low, and the system would seekto operate in the most efficient state of the device to induce thermaltransfer medium flow. This may include laminar flow over the heatdissipating elements of the heatsink. In the next regime, the heatincreases, and as a result, the device to induce thermal transfer mediumflow must increase its flow rate. At this point, a compromise may bemade, between minimum energy cost (and thus a minimization of the energyto drive the device to induce thermal transfer medium flow), andeffective heat dissipation. In this regime, the heatsink may beconfigured to induce turbulence in the medium flow around it. This, inturn, increases the resistance to flow, but reduces the boundary layereffect. Advantageously, in this regime, a fractal physical relationshipof element of the heatsink may act to reduce peak acoustic emission withrespect to frequency. Likewise, by avoiding sharp acoustic resonances,there may be a more effective transfer of heat with lower losses asacoustic energy. Further, the interaction of the elements of theheatsink may be further optimized to achieve higher efficiency. Finally,at maximum heat load, presumably at the limit of the heatsink, thesystem enters a maximum heat dissipation mode. For example, this mode isone traditionally analyzed as the maximum capacity of the heatsink anddevice to induce thermal transfer medium flow system, and as such wouldtypically assume or nearly assume a traditional optimized geometry.However, both due to the fact that the system may include fractalgeometry elements for other regimes of operation, and because these maybe exploited to gain efficiencies over traditional symmetric and regulargeometries, the maximum heart dissipation configuration may be somewhatdifferent than a parallel plate heatsink, for example. Note that not allregions of the heatsink need to operate within the same regime at thesame time, and even under a steady state heat load, may vary cyclically,randomly or chaotically (over a relevant timescale). In this case, theterm “chaotically” is intended to assume its technical meaning underchaos and fractal theory, and not adopt a lay interpretation. On theother hand, “randomly” is intended to encompass true randomness,pseudorandom variations, and deterministic changes that over therelevant timescale have statistical characteristics that modelrandomness within an acceptable margin of error, the acceptabilityrelating to achieving a suitable regime of operation. For example,because some attributes of turbulent flow are random, even though theyare more technically chaotic, the random features may be used toadvantage. For example, the device to induce thermal transfer mediumflow may be subject to a tinsel type flow disruptor, which in someregimes appears to be a random variation in air flow speed, direction,vortex, etc. While this may increase noise, it also can createpersistent disruptions in boundary layers, even on smooth and regularheatsink elements. That is, either the heatsink geometry and the deviceto induce thermal transfer medium flow, or both, may have fractal orchaotic tendencies.

According to one embodiment, the geometry involves branching elements,to increase surface area of the elements. An actuator may be used toalter angles or even to open and close branches. For example, a heatsinkformed of a shape memory alloy (SMA) (such as Nitinol), may be producedby an additive manufacturing process, e.g., a 3D printer or 2.5Dprinter. Such a device may be thermally processed to have characteristicshape changes at temperature transitions, and indeed, the composition ofthe alloy may be controlled during fabrication to produce a variety oftransition temperatures. Therefore, a 3D heatsink may be provided whichinherently changes shape through a series of transitions as thetemperature is increased and decreased. In this embodiment, the changestend to be monotonic with increasing temperature, though by engineeringthe angles and physical configuration, the actual physical shape andheat dissipation properties may assume a non-monotonic function. Notethat in this embodiment, it is generally preferred that only the branchpoints are formed of SMA, and the bulk be formed of a high thermalconductivity material, such as copper and/or silver, or to a lesserextent, aluminum.

According to another embodiment, actuators, which may be SMA, solenoids,or otherwise, are controlled to change the position of repositionableelements. In this case, independent control can be exercised which isnot dependent on temperature, but typically, the number of controlledelements is more constrained due to manufacturing and controlfeasibility issues. The actuators may alter a spacing, angle, position,or engagement of heat sink elements. When a set of regularly spaced andsized elements are controlled according to a constant orspectrally-defined distribution, this can be controlled to operatewithin highly predictable regimes. On the other hand, if the elementsare not regularly sized and spaced, or are controlled in irregularmanner, the resulting fluid dynamics will likely require a statisticalflow (e.g., Monte Carlo) analysis, rather than a simplifying presumptionof static function. This will especially be the case if the thermaltime-constants of the heat flow from the heat source, to the heatsink,and then to the heat transfer fluid, are near or within the range oftime-constants of the turbulence or chaotically varying flows of theheat transfer fluid. Typically, the thermal heat transfer time-constantsare longer than the turbulent or chaotic variation time-constants, andtherefore this meeting this presumption requires either generating lowfrequency turbulent or chaotic variations of the heat transfer fluidmedium, or making the heatsink (and perhaps other elements) with shorttime-constants, such as using short/thin/small elements, using phonontransport phenomena, or other means.

In another embodiment, the time-constant(s) of the thermal transfermedium flow is much shorter than the relevant thermal time-constants ofthe heat source and heatsink, and the purpose of the turbulent orchaotic disruption is to alter the convective heat transfercharacteristics of the heatsink, such as reducing the boundary layers ormaking them dynamically changing over time and space.

Another aspect of the technology involves planar heatsinks, such as usedin antenna designs. In this case, the present technology may havecorresponding effect to that discussed above, especially where a deviceto induce thermal transfer medium flow is provided to cool a generallyplanar heatsink system. It is noted that any heatsink in actuality mustbe considered in three dimensions, and the fact that it may haveexpanses of a thin uniform thickness layer does not defeat use ofthree-dimensional analysis to understand its functioning andoptimization. In the case of a printed circuit board-type heatsink, avariable geometry is typically infeasible. Similarly, there a planarheatsink structure serves a secondary purpose, such as an antenna, thephysical configuration is constrained by this other purpose. However,the device to induce thermal transfer medium flow is typically not soconstrained, and therefore provides a controllable variable. Further, inmany cases, the requirement for “thinness” of a 2D heatsink does notpreclude texturing on an exposed surface, which itself may have afractal variation.

In some cases, a variable geometry may be achieved by altering flowcharacteristics of thermal transfer medium flow, and for example, adeflector may be controlled to change a direction of impingement.Advantageously, a surface of a heatsink can have anisotropic features,which respond differently to different flow direction. Thus, theefficiency of the fan can be optimized other than by fan speed alone, toprovide another control variable. This may have particular importancewhere the fan itself is highly constrained, and cannot simply be madeoversized, or where energy efficiency is an overriding concern.

The technology is not limited to cooling gas fluids, and may encompassliquids. Typically, cooling liquids are recycled, and therefore operatewithin a physically closed system. Use of fractal branching fluidnetworks is known, but various principles discussed above, such asvariable geometry, variations in flow rate over different regimes ofoperation, different directions of flow over surfaces, and intentionalinduction of chaotic flow patterns may be adopted top advantage.

Many fractal designs are characterized by concave regions or cavities.See, for example, FIGS. 2 and 3. While sets of concavities may be usefulin improving aerodynamics and fluid dynamics to increase turbulence, ifthey are disposed in a regular array, they will likely produce anacoustic resonance, and may have peaks in a fluid impedance function. Onthe other hand, the multiscale nature of a fractal geometric design willallow the system to benefit from the concavities, while avoiding anarrowly tuned system.

The present technology proposes, according to one embodiment, afractal-shaped heatsink for the purpose of dissipating heat. Benefits ofa fractal heatsink, over a traditional heatsink having a Euclidiangeometry may include: (1) the fractal heatsink has a greater surfacearea, allowing for more exposure of the hot device to the surroundingair or liquid and faster dissipation of heat; and (2) due to theplethora of concave structures or cavities in fractal structures, thefractal heatsink is better able to take advantage of turbulent flowmechanics than a traditional heatsink, resulting in heat entering andexiting the heatsink more quickly (3) acoustic properties, especially inforced convection systems.

The technology provides, according to various embodiments, a heatsink tocool an object through conduction (diffusion), convection and radiation.See, en.wikipedia.org/wiki/Heat_transfer.

With respect to conduction, the present technology observes that whenheat energy is conducted by phonon transport, wave phenomena arepertinent, and thus a fractal branching network can advantageously beused to reduce reflections at discontinuities and decrease compleximpedance. Further, a fractal geometry may assist in optimizing thecross-section area and surface area (for radiation and convectivetransfer) under given constraints.

With respect to convection, a fractal geometry may provide acousticbenefits, by distributing acoustic energy across a wide band, and thusensuring “whiteness” of a noise spectrum and absence of sharpresonances. Further, a fractal geometry may provide high or maximumsurface area, and produce turbulent cooling medium flows to reduceboundary later effects. Depending on the constraints imposed, a fractalgeometry may also provide chimneys or defined flow paths through anetwork of elements, and thus control an impedance of coolant flow,though generally, a fractal branching network will produce higher flowimpedance than corresponding smooth regular surfaces. In some cases, atextured surface or configuration (as might be achieved by a fractalgeometry) can actually increase laminar flow some distance away from thesurface, by creating a controlled disturbed intermediate layer.

With respect to radiation, a fractal geometry can avoid parallelsurfaces which can limit radiative dissipation. For example, a parallelplate heatsink will radiatively transfer heat between the plates, andthus limit the effectiveness of radiation from the bulk of the surfacesas an effective dissipation mechanism. On the other hand, irregularangles and surface branches may help to avoid reabsorption of thermalradiation by the elements of the heatsink, and thus enhance radiativedissipation.

For the smallest heatsink elements, on the order of 10-100 nm, the focusof the heat transfer may be on radiation rather than convection.Electron emission and ionization may also be relevant. Larger heatsinkelements, approximately >1 mm in size, will generally rely on convectionas the primary form of heat transfer. In a fractal geometry system,elements spanning these regimes may be provided in a single system.

In one embodiment, the heatsink comprises a heat exchange device with aplurality of heat exchange elements having a fractal variationtherebetween. A heat transfer fluid, such as air, water, or another gasor liquid, is induced to flow through the heat exchange device. The heattransfer fluid has turbulent portions. The fractal variation in theplurality of heat exchange elements substantially reduces the narrowband acoustic resonance resulting from fluid flow around the heatsinkelements as compared to a heatsink having a linear or Euclidiangeometric variation between the plurality heat exchange elements. Theturbulent flow also disturbs the stagnant surface boundary layer,leading to more efficient heat transfer, but generally reduced flowrates for the same motive force. Note that, since turbulence dissipatesenergy, under some conditions, the heat added to the system by inducingthe heat transfer fluid flow can be a significant factor.

When a heat transfer fluid (air, gas or liquid) is induced to flow overa surface, there may be turbulence in the fluid. The fractal shape ofthe heatsink would generally provide a range of physical sizeparameters, and thus for any given flow rate, would typically induceturbulent flow over some portion of a fractal geometry array. Notably,because the flow for a given heatsink may vary over a range of speeds,and the temperature and viscosity of the fluid varies over a range ofconditions, a fractal geometry facilitates optimization over a range ofparameters.

In fluid dynamics, turbulence or turbulent flow is a flow regimecharacterized by chaotic property changes. This includes low momentumdiffusion, high momentum convection, and rapid variation of pressure andflow velocity in space and time. See, en.wikipedia.org/wiki/Turbulence;www.scholarpedia.org/article/Turbulence. Flow in which the kineticenergy dies out due to the action of fluid molecular viscosity is calledlaminar flow. While there is no theorem relating the non-dimensionalReynolds number (Re) to turbulence, flows at Reynolds numbers largerthan 5000 are typically (but not necessarily) turbulent, while those atlow Reynolds numbers usually remain laminar. In Poiseuille flow, forexample, turbulence can first be sustained if the Reynolds number islarger than a critical value of about 2040; moreover, the turbulence isgenerally interspersed with laminar flow until a larger Reynolds numberof about 4000. In turbulent flow, unsteady vortices appear on manyscales and interact with each other. Drag due to boundary layer skinfriction increases. The structure and location of boundary layerseparation often changes, sometimes resulting in a reduction of overalldrag. Although laminar-turbulent transition is not governed by Reynoldsnumber, the same transition occurs if the size of the object isgradually increased, or the viscosity of the fluid is decreased, or ifthe density of the fluid is increased. Turbulence is characterized bythe following features: Irregularity: Turbulent flows are always highlyirregular. For this reason, turbulence problems are normally treatedstatistically rather than deterministically. Turbulent flow is chaotic.However, not all chaotic flows are turbulent. Diffusivity: The readilyavailable supply of energy in turbulent flows tends to accelerate thehomogenization (mixing) of fluid mixtures. The characteristic which isresponsible for the enhanced mixing and increased rates of mass,momentum and energy transports in a flow is called “diffusivity”.Rotationality: Turbulent flows have non-zero vorticity and arecharacterized by a strong three-dimensional vortex generation mechanismknown as vortex stretching. In fluid dynamics, they are essentiallyvortices subjected to stretching associated with a correspondingincrease of the component of vorticity in the stretching direction—dueto the conservation of angular momentum. In general, the stretchingmechanism implies thinning of the vortices in the directionperpendicular to the stretching direction due to volume conservation offluid elements. As a result, the radial length scale of the vorticesdecreases and the larger flow structures break down into smallerstructures. The process continues until the small-scale structures aresmall enough that their kinetic energy can be transformed by the fluid'smolecular viscosity into heat, i.e., atomic scale random motion. This iswhy turbulence is always rotational and three dimensional. Dissipation:To sustain turbulent flow, a persistent source of energy supply isrequired because turbulence dissipates rapidly as the kinetic energy isconverted into internal energy by viscous shear stress. It thereforebecomes apparent that, because turbulent flow is chaotic, anoptimization of heatsink geometry responsive to chaotic features canachieve efficiencies over a range of operating regimes, and atparticular operating regimes.

Turbulence causes the formation of eddies of many different lengthscales. Most of the kinetic energy of the turbulent motion is containedin the large-scale structures. The energy “cascades” from theselarge-scale structures to smaller scale structures by an inertial andessentially inviscid mechanism. This process continues, creating smallerand smaller structures which produces a hierarchy of eddies. Eventuallythis process creates structures that are small enough that moleculardiffusion becomes important and viscous dissipation of energy finallytakes place. The scale at which this happens is the Kolmogorov lengthscale.

Via this energy cascade, turbulent flow can be realized as asuperposition of a spectrum of flow velocity fluctuations and eddiesupon a mean flow. The eddies are loosely defined as coherent patterns offlow velocity, vorticity and pressure. Turbulent flows may be viewed asmade of an entire hierarchy of eddies over a wide range of length scalesand the hierarchy can be described by the energy spectrum that measuresthe energy in flow velocity fluctuations for each length scale(wavenumber). The scales in the energy cascade are generallyuncontrollable and highly non-symmetric. Nevertheless, based on theselength scales these eddies can be divided into three categories.

Integral length scales: Largest scales in the energy spectrum. Theseeddies obtain energy from the mean flow and also from each other. Thus,these are the energy production eddies which contain most of the energy.They have the large flow velocity fluctuation and are low in frequency.Integral scales are highly anisotropic. The maximum length of thesescales is constrained by the characteristic length of the apparatus.

Kolmogorov length scales: Smallest scales in the spectrum that form theviscous sub-layer range. In this range, the energy input from nonlinearinteractions and the energy drain from viscous dissipation are in exactbalance. The small scales have high frequency, causing turbulence to belocally isotropic and homogeneous.

Taylor microscales: The intermediate scales between the largest and thesmallest scales which make the inertial subrange. Taylor microscales arenot dissipative scale but pass down the energy from the largest to thesmallest. Taylor microscales play a dominant role in energy and momentumtransfer in the wavenumber space.

The Russian mathematician Andrey Kolmogorov proposed the firststatistical theory of turbulence, based on the aforementioned notion ofthe energy cascade (an idea originally introduced by Richardson) and theconcept of self-similarity (e.g., fractal relationships). For very highReynolds numbers, the small-scale turbulent motions are statisticallyisotropic (i.e. no preferential spatial direction could be discerned).In general, the large scales of a flow are not isotropic, since they aredetermined by the particular geometrical features of the boundaries (thesize characterizing the large scales will be denoted as L). Thus,Kolmogorov introduced a second hypothesis: for very high Reynoldsnumbers the statistics of small scales are universally and uniquelydetermined by the kinematic viscosity (v) and the rate of energydissipation (ε). With only these two parameters, the unique length(Kolmogorov length scale) that can be formed by dimensional analysis is

$\eta = {\left( \frac{V^{3}}{ɛ} \right)^{1/4}.}$

A turbulent flow is characterized by a hierarchy of scales through whichthe energy cascade takes place. Dissipation of kinetic energy takesplace at scales of the order of Kolmogorov length η, while the input ofenergy into the cascade comes from the decay of the large scales, oforder L. These two scales at the extremes of the cascade can differ byseveral orders of magnitude at high Reynolds numbers. In between thereis a range of scales (each one with its own characteristic length r)that has formed at the expense of the energy of the large ones. Thesescales are very large compared with the Kolmogorov length, but stillvery small compared with the large scale of the flow (i.e., η<<r<<L).Since eddies in this range are much larger than the dissipative eddiesthat exist at Kolmogorov scales, kinetic energy is essentially notdissipated in this range, and it is merely transferred to smaller scalesuntil viscous effects become important as the order of the Kolmogorovscale is approached. Within this range inertial effects are still muchlarger than viscous effects, and it is possible to assume that viscositydoes not play a role in their internal dynamics (for this reason thisrange is called “inertial range”). Kolmogorov theory is at present underrevision. The theory implicitly assumes that the turbulence isstatistically self-similar at different scales. This essentially meansthat the statistics are scale-invariant in the inertial range. However,there is evidence that turbulent flows deviate from this idealizedbehavior. See, Davidson, P. A. (2004). Turbulence: An Introduction forScientists and Engineers. Oxford University Press. ISBN978-0-19-852949-1; scholarpedia.org; G. Falkovich, Scholarpedia,“Cascade and scaling”; Jin, Y.; Uth, M.-F.; Kuznetsov, A. V.; Herwig, H.(2 Feb. 2015). “Numerical investigation of the possibility ofmacroscopic turbulence in porous media: a direct numerical simulationstudy”. Journal of Fluid Mechanics 766: 76-103. Bibcode:2015JFM . . .766 . . . 76J. doi:10.1017/jfm.2015.9; G Falkovich and K. R.Sreenivasan. Lessons from hydrodynamic turbulence, Physics Today, vol.59, no. 4, pages 43-49 (April 2006); J. Cardy, G. Falkovich and K.Gawedzki (2008) Non-equilibrium statistical mechanics and turbulence.Cambridge University Press; P. A. Durbin and B. A. Pettersson Reif.Statistical Theory and Modeling for Turbulent Flows. Johns Wiley & Sons,2001; T. Bohr, M. H. Jensen, G. Paladin and A. Vulpiani. DynamicalSystems Approach to Turbulence, Cambridge University Press, 1998; J. M.McDonough (2007). Introductory Lectures on Turbulence—Physics,Mathematics, and Modeling; Kolmogorov, Andrey Nikolaevich (1941). “Thelocal structure of turbulence in incompressible viscous fluid for verylarge Reynolds numbers”. Proceedings of the USSR Academy of Sciences (inRussian) 30: 299-303, translated into English by V. Levin: Kolmogorov,Andrey Nikolaevich (Jul. 8, 1991). Proceedings of the Royal Society A434 (1991): 9-13. Bibcode:1991RSPSA.434 . . . 9K.doi:10.1098/rspa.1991.0075; Kolmogorov, Andrey Nikolaevich (1941).“Dissipation of Energy in the Locally Isotropic Turbulence”. Proceedingsof the USSR Academy of Sciences (in Russian) 32: 16-18, translated intoEnglish by Kolmogorov, Andrey Nikolaevich (Jul. 8, 1991). Proceedings ofthe Royal Society A 434 (1980): 15-17. Bibcode:1991RSPSA.434 . . . 15K.doi:10.1098/rspa.1991.0076; G. K. Batchelor, The theory of homogeneousturbulence. Cambridge University Press, 1953.

Therefore, the efficiency of heat transfer may be increased as comparedto a heat exchange device having a linear or Euclidian geometricvariation between several heat exchange elements, at least over certainregimes of operation.

The heat exchange device may include a highly conductive substance whoseheat conductivity exceeds 850 W/(m·K). Examples of such superconductorsinclude graphene, diamond, and diamond-like coatings. Alternatively, theheat exchange device may include carbon nanotubes. At such high thermalconductivities, phonon heat transport may be at play.

A heatsink according to the present technology may be manufactured, forexample, by additive manufacturing (e.g., 3D print), casting, orsubtractive manufacturing (machining). Further, a cast design may beproduced by a lost wax or lost foam design from a 3D printed form ortemplate. Thus, in practice, a design is generated on a computer-aideddesign (CAD) system, which may, for example, employ algorithms tooptimize the shape according to various criteria, such as size, weight,heat load, air flow, other convective heat transfer parameters, infraredradiation recapture, and other criteria. The design is then converted,by a computer assisted manufacturing (CAM) system, such as an additivemanufacturing “3D” printer or 2.5D printer (layers), into a form. Theform, if produced using a metal sintering or ceramic process, may itselfbe a heatsink, though more typically the form is a polymer, which canthen be used to create a mold. The mold, in turn, can be used to createmultiple templates, which can be used in a casting process. As a result,relatively complex mechanical designs can be replicated in volume. Insome cases, when the heatsink is molded, the metal may be heterogeneous,resulting in a range of properties over different regions of the mold.

The design, in some cases, will result in a fractal shape, which mayhave branches or multiple levels of branches, with multiplecharacteristic scales, which may have some symmetries or repetitions, orbe absent symmetries and repetitions. A design which lacks symmetries orrepetitions, and is self-similar at various scales, is considered“fractal”. A design which adopts some of these characteristics, orfunctionally emulates some of these characteristics, is considered“fractal-like”. A design representing an array of uniform, repeatingelements of the same scale is generally considered non-fractal. In somecases, a branching array having multi-way symmetry may in some cases beconsidered fractal-like. A multiscale fractal (i.e., with asymmetrieswithin each scale range) with outwardly tapering branches will tend tocarry and dissipate heat further from the heat source than a symmetricdesign, since by nature the larger cross section branches will carryhear further than their smaller, higher surface area per mass cousinbranches, and the asymmetry will tend to assure that some branchesindeed have larger cross sections; however, this is not the only effectto be obtained. Since the fractal is typically generated by an iterativefunction system (IFS) responsive to its local environment, the fractalmay be optimized by a steering function to steer heat flow to areas withhighest convective heat loss, while avoiding heat flow toward brancheswhich do not efficiently shed heat. Similarly, in a vacuum heatsinkemitter, the heat loss tends to be radiative, and the optimization canaddress maximization of net radiative heat loss within the constrainedenvironment.

The present technology, in an ambient atmosphere, may be subject to dustor fiber buildup due to particulates in the flow of cooling air.Filtering of the air to completely remove such particulates isinefficient, since the required filter would require significant energyto operate, and that energy both increases the heat load of theaggregate system to be shed, increases power consumption, and presents acompromise with respect to use of the same energy of more globally,system manufacturing and operating cost, that could be reallocated to anet higher efficiency, such as a heatsink with less susceptibility todust or fiber deposition and a higher cooling air flow rate. However,the dust deposition may be modelled, and included within a designequation, e.g., an iterative function system, for generating an optimalheatsink which may have resulting fractal or fractal-like features.

As discussed herein, there are a number of strategies available toremove dust that has accumulated on the heatsink surfaces, and thesystem, including the heat source, heatsink, dust, air flow (e.g., fan)system, as well as the dust abatement system, may be together modelled.This model will typically have a time variance, and the operating pointof the aggregate system may change over time, especially if the dustabatement system operates discontinuously. In such a system, the heatflow vectors within the heatsink may change over time in relativemagnitude to each other, and the design system therefore typicallymodels the system over its range of operation. In one embodiment, a fancontroller (typically the only controllable part of the heatsink) may becontrolled based not simply on a temperature and/or temperature riserate of the heatsink, but also a convective and fluid dynamic model ofthe system, including measured or estimate dust, fiber, debris, etc. Thefan controller may in some cases speed up the fan in an attempt to blowoff dust, or create turbulence to disrupt dust, or to createvelocity/pressure gradient dependent flow patterns around the heatsinkto achieve efficient and/or optimal heat transfer. Maintaining lowoperating temperatures of the heat source and energy cost are notnecessarily the only critical variables, and therefore in some cases,the fan will run at a fan speed which is energy-inefficient with respectto the lowest speed (lowest energy cost) that will achieve the desiredcooling of the heat source.

The controller may also implement an acoustic/sonic model, especiallywhere turbulent air flow is intentionally created, and the model may beused to ensure that acoustic emissions are not objectionable or outsideof a predetermined or adaptive limit. See, U.S. Pat. No. 6,850,252.Likewise, in some cases, the sounds emitted by the heatsink system maybe intentionally timed to external cues.

Various variations on this heatsink will be apparent to skilled personsin the art. For example, the heatsink could include a heat transfersurface that is connected to the heat exchange device and is designed toaccept a solid to be cooled. Alternatively, there could be a connectorthat is designed to connect with a solid to be cooled in at least onepoint. In another embodiment, there are at least three connectorsserving to keep the solid and the heatsink in a fixed position relativeto one another. Various connectors will be apparent to persons skilledin the art. For example, the connector could be a point connector, abus, a wire, a planar connector or a three-dimensional connector. Inanother embodiment, the heatsink has an aperture or void in the centerthereof designed to accept a solid to be cooled. The heatsink may alsobe integral to the heat source, or attached by other means.

This heatsink is typically intended to be used to cool objects, and maybe part of a passive or active system. Modern three-dimensional laserand liquid printers can create objects such as the heatsinks describedherein with a resolution of features on the order of about 16 μm, makingit feasible for those of skilled in the art to use such fabricationtechnologies to produce objects with a size below 25 cm. Alternatively,larger heatsinks, such as car radiators, can be manufactured in atraditional manner, designed with an architecture of elements having afractal configuration. For example, a liquid-to-gas heat exchanger(radiator) may be provided in which segments of fluid flow conduit havea fractal relationship over three levels of recursion, i.e., paths withan average of at least two branches. Other fractal design concepts maybe applied concurrently, as may be appropriate.

Yet another embodiment of the invention involves a method of cooling asolid by connecting the solid with a heatsink. The heatsink comprises aheat exchange device having a plurality of heat exchange elements havinga fractal variation therebetween. A heat transfer fluid having turbulentportions is induced to flow with respect to the plurality of heatexchange elements. The fractal variation in the plurality of heatexchange elements serves to substantially reduce narrow band resonanceas compared to a corresponding heat exchange device having a linear orEuclidean geometric variation between a plurality of heat exchangeelements.

A preferred embodiment provides a surface of a solid heatsink, e.g., aninternal or external surface, having fluid thermodynamical propertiesadapted to generate an asymmetric pattern of vortices over the surfaceover a range of fluid flow rates. For example, the range may comprise arange of natural convective fluid flow rates arising from use of theheatsink to cool a heat-emissive object. The range may also comprise arange of flow rates arising from a forced convective flow (e.g., a fan)over the heatsink.

The heatsink may cool an unconstrained or uncontained fluid, generallyover an external surface of a heatsink, or a constrained or containedfluid, generally within an internal surface of a heatsink.

It is therefore an object of the present invention to provide a heatsinksystem comprising: a base structure configured to interface with a heatsource; a heat exchange device configured to receive heat from the basestructure, and emit the received heat from a heat exchange surface, intoan external surrounding heat exchange medium, the heat exchange surfacebeing subject to accumulation of particles; and a particle dislodgingdevice configured to mechanically disrupt an accumulation of particleson the plurality of heat exchange elements.

It is also an object of the present invention to provide a method ofheat transfer, comprising: providing a base structure configured tointerface with a heat source; receiving heat from the base structurewith a heat exchange device configured to emit the received heat from aheat exchange surface, into an external surrounding heat exchangemedium; and reducing an accumulation of particles on the heat exchangesurface with at least one of a particle degrading device and a particledislodging device.

It is a further object of the present invention to provide a heatsinkcomprising: a base structure configured to interface with a heat source;a heat exchange device configured to receive heat from the basestructure, and emit the received heat from a heat exchange surface, intoan external surrounding heat exchange medium, the heat exchange surfacebeing subject to accumulation of particles; and a particle degradingdevice configured to chemically degrade an accumulation of particles onthe plurality of heat exchange elements.

It is a still further object of the present invention to provide asystem comprising: a fractal heat exchange device comprising: a basestructure configured to interface with a heat source; a plurality ofheat exchange elements having approximately fractal geometry, theplurality of heat exchange elements attached to the base structure,configured to receive heat from the base structure and emit the heatinto an external surrounding through radiation and convection in heatexchange medium; and a pyrolizer to pyrolize dust particles.

Another object of the present invention provides a method of heattransfer comprising providing a base structure configured to interfacewith a first heat source; receiving heat from the base structure with afractal heat exchange device configured to emit the received heat from aplurality of heat exchange elements, into an external surrounding heatexchange medium; providing a second heat source distant from the firstheat source, the second heat source used to heat dust particles in avicinity of the first heat source; pyrolizing dust particles using heatfrom the second heat source; and dissipating the heat used to pyrolizedust particles.

A still further object of the present invention provides a fractal heatexchange device comprising: a base structure configured to interfacewith a heat source; a plurality of heat exchange elements havingapproximately fractal geometry, the a plurality of heat exchangeelements attached to the base structure and configured to receive heatfrom the base structure and emit the heat into an external surroundingthrough radiation and convection in heat exchange medium; and a vibratorto vibrate at least a subset of the plurality of heat exchange elementsto dislodge dust particles from heat exchange elements, wherein the basestructure comprises vibration isolator to prevent vibrations fromdamaging the heat source. The vibrator may be one of a piezoelectrictransducer and electromagnetic transducer. The vibration isolator may beone of a plastic thermal transfer medium, a non-shear transmissive solidand an active vibration suppression interface. The non-sheartransmissive solid may be a copper wire bundle. The base structure mayfurther comprise an anisotropic vibration transmissive mount to isolatevibrations from the heat source. The anisotropic vibration transmissivemount may comprise a piston in a cylinder.

Another object of the present invention provides a method of heatexchange comprising: providing a base structure configured to interfacewith a heat source; receiving heat from the base structure with a heatexchange device configured to emit the received heat from a plurality ofheat exchange elements, into an external surrounding heat exchangemedium; providing a source of vibration to vibrate the plurality of heatexchange elements; vibrating the plurality of heat exchange elements todislodge dust particles therefrom; and dissipating vibrations beforethey reach the heat source.

It is also an object of the present invention to provide a method heatexchange comprising: providing a base structure configured to interfacewith a heat source; receiving heat from the base structure with a heatexchange device configured to emit the received heat from a plurality ofheat exchange elements, into an external surrounding heat exchangemedium; providing a time-varying flow of the heat exchange medium overthe plurality of heat exchange elements; and dislodging dust particlesaccumulated on the plurality of heat exchange elements.

It is a still further object of the present invention to provide afractal heat exchange device comprising: a base structure configured tointerface with a heat source; a plurality of heat exchange elementshaving approximately fractal geometry, the a plurality of heat exchangeelements attached to the base structure and being configured to receiveheat from the base structure and emit the heat into an externalsurrounding through radiation and convection in heat exchange medium; anelectrostatic charge generator; and an electrostatic discharge device,wherein the electrostatic charge generator is configured to inducestatic electricity on a surface of at least a portion of the pluralityof heat exchange elements to repel dust particles from accumulatingthereon.

It is another object of the present invention to provide a method ofheat exchange comprising: providing a base structure configured tointerface with a heat source; receiving heat from the base structurewith a heat exchange device configured to emit the received heat from aplurality of heat exchange elements, into an external surrounding heatexchange medium; inducing a first static electric charge having apolarity on the surface of at least a portion of the a plurality of heatexchange elements; and inducing a second static electric charge on dustparticles, the second static electric charge having the same polarity asthe polarity of the first static electric charge.

Another object of the present invention is to provide a systemcomprising: a fractal heat exchange device, the heat exchange devicefurther comprising: a base structure configured to interface with a heatsource; a plurality of heat exchange elements having approximatelyfractal geometry, the plurality of heat exchange elements being attachedto the base structure and being configured to receive heat from the basestructure and emit the heat into an external surrounding heat transfermedium by radiation and convection; and at least one of a fan and acompressor, configured to induce a time-varying flow of the heattransfer medium over the plurality of heat exchange elements, wherein atleast portions of the time varying flow of the heat transfer medium overthe plurality of heat exchange elements are turbulent, having aturbulence pattern that changes over time.

The particle-dislodging device may comprise a vibrator configured tovibrate a plurality of heat exchange elements comprising the heatexchange surface. The particle-dislodging device may also comprise atleast one of a piezoelectric transducer and an electromagnetictransducer. The particle-dislodging device may also comprise a rotatingmotor configured to induce a vibration in the plurality of heat exchangeelements. The particle-dislodging device may also comprise a fan or pumpconfigured to induce a time-varying flow of heat exchange media over theplurality of heat exchange elements. The time-varying flow of heatexchange media may comprise entrained particles or liquid, e.g., agas-liquid mixture. The particle-dislodging device may further comprisean electrostatic charge generator. The particle-dislodging device mayalso comprise an electrostatic discharge device. The particle-dislodgingdevice may comprise at least one shape memory alloy or a bimetalelement. The particle dislodging system may comprise anelectrical-vibration transducer and an oscillating signal generator,receiving a feedback signal from the feedback transducer, configured toexcite the vibration transducer. The particle-dislodging device maycomprise a fan or compressor, configured to induce a flow of a gaseousheat transfer medium over a plurality of heat exchange elements of theheat exchange surface. The particle dislodging device may comprise a fanor compressor, configured to induce a flow of a gaseous heat transfermedium over the heat exchange surface along at least one vector, havingat least one control input, wherein the at least one vector is alteredin dependence on the at least one control input.

The system may further comprise at least one a vibrational transducer,controlled to cancel vibrations at the base structure produced by theparticle-dislodging device. A vibration damper may be provided,configured to damp vibrations at the base structure. A feedbacktransducer may be provided, configured to detect vibrations.

The heat exchange surface may comprise a plurality of heat exchangeelements having resonances over a range of frequencies, and theparticle-dislodging device comprises an electrical-vibration transducerand an oscillating signal generator, configured to generate vibrationsover the range of frequencies, to resonate the plurality of heatexchange elements. The heat exchange surface may also comprise aplurality of heat exchange elements having characteristic dimensionsover at least two orders of size scales. The heat exchange surface maycomprise a plurality of heat exchange elements, and theparticle-dislodging device may comprise an actuator configured to alterat least one spatial relationship of a first of the plurality of heatexchange elements with respect to a second of the plurality of heatexchange elements. The actuator may be a passively activated memberresponsive to temperature. The actuator may also be actively controlledby an automated electronic processor in dependence on a computationalheat exchange model of the heatsink system.

The accumulation of particles on the heat exchange element may bereduced with a particle-degrading device. The particle-degrading devicemay comprise a pyrolizer, degrading the particles by pyrolysis. Theparticle-degrading device may also comprise a pump configured to cause atime varying flow of a liquid solvent entrained in a gas heat exchangemedium on the heat exchange surface. The particle-degrading device maycomprise a laser. The particle-degrading device may comprise anelectrical discharge plasma emitter.

The accumulation of particles on the heat exchange surface may bereduced by vibration. The accumulation of particles on the heat exchangesurface may be reduced with a piezoelectric transducer. The accumulationof particles on the heat exchange surface may be reduced with anelectromagnetic transducer. The accumulation of particles on the heatexchange surface may be reduced with a rotating motor configured toinduce a vibration in the plurality of heat exchange elements. Theaccumulation of particles on the heat exchange surface may be reducedwith at least one active system, which induces a time-varying flow ofheat exchange media over the heat exchange surface. The time-varyingflow of heat exchange media may comprise entrained particles. Thetime-varying flow of heat exchange media may comprise a liquid mixedwith a gas. The method may further comprise inducing a flow of a gaseousheat transfer medium comprising an entrained solvent over the pluralityof heat exchange elements. The accumulation of particles on the heatexchange surface may be reduced by generating an electrostatic charge.The accumulation of particles on the heat exchange surface may bereduced by use of an electrostatic discharge generator. The accumulationof particles on the heat exchange surface may be reduced by heating andcooling at least one shape memory alloy or bimetal element. Theaccumulation of particles on the heat exchange surface may be reduced byselectively activating a laser. The accumulation of particles on theheat exchange surface may be reduced by inducing transient thermalchanges proximate to the heat exchange surface. The accumulation ofparticles on the heat exchange surface may be reduced by selectivelygenerating vibrations with a vibrational transducer, controlled tocancel vibrations at the base structure. The accumulation of particleson the heat exchange surface may be reduced by inducing vibrations in aplurality of heat exchange elements of the heat exchange surface, anddamping vibrations at the base structure. The accumulation of particleson the heat exchange surface may be reduced by inducing a flow of agaseous heat transfer medium over the heat exchange elements along atleast one vector, having at least one control input, wherein the atleast one vector is altered in dependence on the at least one controlinput.

The method may further comprise detecting vibrations with a feedbacktransducer, and generating vibration with an electrical-vibrationtransducer in dependence on a signal received from the feedbacktransducer. The heat exchange surface comprises a plurality of heatexchange elements having resonances over a range of frequencies, themethod further comprising generating vibrations over the range offrequencies, to resonate the plurality of heat exchange elements.

The heat exchange surface may comprise a plurality of heat exchangeelements having characteristic dimensions over at least two orders ofsize scales. The plurality of heat exchange elements may havefractal-like features, or have fractal-like relationships with eachother. The heat exchange surface may comprise a plurality of heatexchange elements, and the accumulation of particles on the heatexchange surface may be reduced by altering at least one spatialrelationship of a first of the plurality of heat exchange elements withrespect to a second of the plurality of heat exchange elements with anactuator. The actuator may be a passively activated member responsive totemperature. The actuator may be actively controlled by an automatedelectronic processor in dependence on a computational heat exchangemodel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a set of governing equations for a parallel plate heatsink.

FIG. 2 illustrates a fractal heatsink that is an exemplary embodiment ofthe invention. In this embodiment, the heatsink is placed adjacent tothe object to be cooled.

FIG. 3 illustrates a fractal heatsink that is an exemplary embodiment ofthe invention. In this embodiment, the heatsink is placed eitheradjacent to or surrounding the object to be cooled.

FIG. 4 illustrates a fractal heatsink that is an exemplary embodiment ofthe invention. In this embodiment, the heatsink is based on a QuadraticKoch Island.

FIG. 5A illustrates the basis for the Quadratic Koch Island.

FIG. 5B illustrates a Quadratic Koch Island obtained after applicationof one iteration.

FIG. 5C illustrates a Quadratic Koch Island obtained after applicationof several iterations.

FIG. 6 illustrates the total length of all the fractal segments of aQuadratic Koch Island.

FIG. 7A illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on amodified Koch Snowflake.

FIG. 7B illustrates the basis for generating the modified Snowflake.

FIG. 8A illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on aSierpinski Carpet.

FIG. 8B illustrates the basis for generating the Sierpinski Carpet.

FIG. 9 illustrates a fractal heatsink that is an exemplary embodiment ofthe invention. In this embodiment, the heatsink is based on a Mandelbox.

FIG. 10 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on aSierpinski tetrahedron.

FIG. 11 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on aDodecaedron fractal.

FIG. 12 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on aIcosahedron flake.

FIG. 13 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on anOctahedron flake.

FIG. 14 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on a 3DQuadtratic Koch.

FIG. 15 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on aJerusalem cube.

FIG. 16 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on a vonKoch surface.

FIG. 17 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on a Mengersponge.

FIG. 18 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on a 3D Hfractal.

FIG. 19 illustrates a fractal heatsink that is an exemplary embodimentof the invention. In this embodiment, the heatsink is based on aMandelbulb.

FIGS. 20-37 illustrate various heatsink designs and proposals, which maybe used in conjunction with various embodiments of the technology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 illustrates a heatsink implementing a first embodiment of thisinvention. Note that the illustration is in two dimensions, but athree-dimensional embodiment is both possible and preferred. There is aheat transfer surface 100 that allows the heatsink to rest comfortablyon a surface, such as the solid to be cooled 190. In the illustratedembodiment, the heat transfer surface 100 is roughly planar, having aclosed Euclidian cross-section on the bottom. However, it might alsohave another shape, for example if the solid to be cooled does not havea planar face. The heat transfer surface may also comprise ananisotropic vibration transfer thermal interface material, such as abraided or straight fine copper wire bundle. Such a bundleadvantageously has strands of different length, which, for example,could permit destructive interference of vibrations transmitted alongeach strand. A fractal-shaped heat exchange device begins at point 110.The base of the fractal-shaped heat exchange device at point 110 mayalso or alternately include a piston and cylinder, to providevibrational isolation along the piston movement axis, while alsotransmitting heat from the heat source to the heatsink along theperipheral wall of the cylinder to the inner wall of the piston. Theworking fluid within the cylinder may be a heat transfer fluid, and aset of valves may be actuated based on the vibration to induce a flow.The heat transfer fluid may be a phase change fluid, and the gaseousphase may vent from the cylinder through a valve. Note that the heatsinkhas three branches leaving from point 110—branch 120, branch 140, andbranch 160. Also note that the branch structure initiating from point110 is nearly identical to that at point 122 and 142, even though onlypoint 110 is a true starting point. Thus, the fractal property ofself-similarity is present. We call the structure that begins at point110 the “first motif,” the structure from point 122 the “second motif,”and the structure that begins from point 142 the “third motif.” Notethat, in the embodiment illustrated in FIG. 2, the replication fromfirst to second motif and from second to third motif involves a lineardisplacement (upward) and a change of scale. In branches not going inthe same direction as the prior branch, there is also a rotation. Underthe limitations for ideal fractals, the second motif and third motif area smaller, similar copy of the first motif. However, due to thelimitations imposed by human-made structures and machines, the fractalsdesigned here are generally finite and the second motif will thus be aninexact copy of the first motif, i.e. if there are N levels startingfrom the first motif, the second motif level will have N−1 levels, if Nis very large, the difference is insignificant. In other words, theself-similarity element required in fractals is not preserved perfectlyin the preferred designs due to the limitations of available machinery,other feasibility constraints, and various design issues. In addition,the benefits are achieved without requiring fractal relationships overmore than a few “orders” of magnitude (iterations of the fractalrecursive algorithm). For example, in the embodiment illustrated in FIG.2, there are no continuing branch divisions and iterations at point 162,even though an ideal fractal would have them. In an ideal fractal, therewould be an infinite number of sub-branches from 110, 122, and 142.However, an imperfect fractal shape, as illustrated in FIG. 2, willserve the purposes of this invention.

FIG. 2 shows various embodiments of the invention, which may be usedindividually, in combination, or in subcombination. When ambient airflows over a textured surface, such as a branching fractal orfractal-like shape, dust, fibers and/or debris may accumulate. Inaddition, in some cases, pollutants or oils may deposit. Suchdepositions tend to reduce the efficiency of heat transfer, and whensufficiently thick, should be removed or disrupted. According to oneembodiment, a particle-dislodging device configured to mechanicallydisrupt an accumulation of particles on the plurality of heat exchangeelements is provided, e.g., vibration transducer 126, fan 127, actuator132, etc.

Thus, for example, the particle-dislodging device may comprise avibration transducer 126 configured to vibrate a plurality of heatexchange elements comprising the heat exchange surface. The vibrationtransducer 126 may be, for example, a piezoelectric transducer, anelectromagnetic transducer, or a rotating motor. In the case of avibration transducer, it is preferred that the vibrations be emitted atresonant frequencies of the heat exchange elements; which advantageouslyspan a range due to the fractal or fractal-like disposition. Therefore,vibrational energy can be selectively targeted to certain elements,without resonant vibration of the entire structure. The vibrationalenergy may be controlled to scan a range of frequencies, or to targetspecific frequencies corresponding to targeted structures.

The system may further comprise at least one vibrational transducer 130,controlled by a feedback-controlled vibration generator 128 to cancelvibrations at the base structure produced by the particle dislodgingdevice 126, based on a signal from a vibration sensing transducer 131.

A vibration damper may be provided to damp vibrations at the basestructure, e.g., near the point 110. This may be an isotropic oranisotropic vibration isolator, and for example may comprise a bundle ofwires (e.g., copper), a piston and cylinder, a particle-filled polymericthermal interface material, copper nanotubes, or the like.

A fan 126 or a heat transfer fluid (which may be gaseous or liquid)pump/compressor may be provided, which in turn may be controlled bye.g., motor speed control 128 to induce a time-varying flow of heatexchange media over the plurality of heat exchange elements. The fan 126or pump/compressor may be configured to induce a flow of a gaseous heattransfer medium over the heat exchange surface along at least onevector, having at least one control input, wherein the at least onevector is altered in dependence on the at least one control input, by,for example a set of louvers 137. The flow rate may also be controlledover time, in dependence on thermal load, desired turbulence or otherconvective heat transfer phenomenon, acoustic emissions, or othercriteria.

The heat exchange media may comprise entrained particles 125, such asmagnetic particles 125, which impinge on the surfaces of the heatexchange elements, and can dislodge surface debris. Advantageously, amagnetic collector can capture the particles for reuse, after mixeddebris is separated. The entrained particles 125 may also be liquiddroplets in a gas-liquid mixture.

In an alternate embodiment, the particle-dislodging device comprises anelectrostatic charge generator and an electrostatic discharge device129. These cooperate to charge the surfaces of the heat exchanger, whichin conjunction with a collection plate/discharge device, induce a forceon the surface particles to move from the heat exchange surface to thecollection plate.

The particle-dislodging device may also comprise a shape memory alloy132 or bimetal element, which for example is passively controlled by atemperature, or actively controlled by control 133 to change theconfiguration of the heatsink. The control 133 may include an automatedelectronic processor in dependence on a computational heat exchangemodel of the heatsink system. Other types of actuator configured toalter at least one spatial relationship of a first portion of the heatexchange elements with respect to a second portion of the heat exchangeelements are possible.

In an alternate embodiment, the particle-degrading device is configuredto chemically degrade an accumulation of particles on the plurality ofheat exchange elements. For example, the particle-degrading device maybe a pyrolizer 134, discharge plasma emitter 136, solvent wash (solventas entrained particles 125), etc. These chemical degradation effectsneed not be constant, and can thus vary in intensity, duty cycle, etc.over time.

A laser 135 may be provided to ablate or disrupt the accumulation. Thelaser may be, for example, controlled by electronically controlledmirrors. On some cases, a continuous scanning is desired, and thecontrol may be a simple area scan of a pulsed laser beam.

The fractal heatsink has a much larger surface area than the heattransfer surface alone, or a regular array of heatsink because all ofthe “branches” and “leaves” of the fern-like fractal shape serve toincrease the surface area. In addition, if a heat transfer fluid isinduced to flow above the heat transfer surface 100, the turbulentportions of the heat transfer fluid near the surface will be increasedby the textures inherent in the fractal variation in the heat exchangeelement 110. Because the fractal patterns is itself non-identicallyrepeating within the fractal design, this will serve to substantiallyreduce narrow band acoustic resonance as compared to a correspondingheat exchange device having a repeating design, e.g., a linear orgeometric variation between several heat exchange elements, therebyfurther aiding in the heat transfer process.

In a preferred embodiment, the heat transfer surface 100 and the roughlyfractal-shaped heat exchange element 110 are all made out of anefficient heat conductor, such as copper or aluminum, or morepreferably, having a portion whose heat conductivity exceeds 850W/(m*K), such as graphene with a heat conductivity of between 4840 and5300 W/(m*K) or diamond with a heat conductivity between 900 and 2320W/(m*K). This would allow heat to quickly enter the heatsink from thesolid and for heat to quickly exit the heatsink through the branches andleaves of the fern-like fractal 110. In another embodiment, the heatsinkis formed, at least in part, of carbon nanotubes, which displayanisotropic heat conduction, with an efficient heat transfer along thelong axis of the tube. Carbon nanotubes are submicroscopic hollow tubesmade of a chicken-wire-like or lattice of carbon atoms. These tubes havea diameter of just a few nanometers and are highly heat conductive,transferring heat much faster than diamond, and in some cases comparableto graphene. See news.mit.edu/2010/thermopower-waves-0308.

Also note that this exemplary embodiment provides a plethora ofopenings, e.g. 124 and 126, between the branches or fractal sub-elementsto ensure that all of the branches are exposed to the surrounding air,gas or liquid and to allow the heat to escape from the heatsink into thesurroundings. In one embodiment of the invention, at least two of theseopenings are congruent, as are openings 124 and 126 illustrated here. Anembodiment of the invention allows the openings to be filled with theair or liquid from the surrounding medium. Due to the limitation imposedby the solid's flat shape, it is not possible to increase the exposureof the fern-like fractal to the solid. However, the air or liquidoutside of the solid are perfect for the fractal's exposure.

Under the phonon model of heat exchange, applicable to carbon nanotubes,graphene materials, and perhaps others, the fractal shape isadvantageous to ensure the escape of the phonons into the surroundingfluid medium because the fractal configuration may avoid peaked internalreflection of waves, and provide high surface exposure to the fluid heattransfer medium. Skilled persons in the art will realize that this couldbe achieved through many known structures. For example, graphene, whichis one-atom-thick carbon and highly heat conductive, would be anadvantageous material to use to build a 2D implementation of the fractalheatsink herein described.

When a turbulently flowing fluid passes around an obstacle, concaveregions or cavities in the obstacle create pockets of separated flowwhich generates self-sustaining oscillations and acoustic resonance.Convex regions may also be provided. These regions may be provided in afractal arrangement. In this aspect of the technology, fractal is meantto signify self-similar but with differences in scale and optionallyanother attribute. The regions may produce substantially reduced narrowband acoustic resonance as compared to regularly spaced and arrangeddisruptions in the flow pattern. Likewise, the presence of disruptionsdisturbs the surface layer and may enhance convective heat transfer.

FIG. 3 illustrates another embodiment of the invention. A solid to becooled that has an arbitrary shape 290 is located inside (illustrated)or outside (not illustrated) a two-dimensional or three-dimensionalroughly fractal shaped 210 heatsink. In one embodiment, the heatsink 210has an aperture 270 designed to hold the solid. Note that, as in FIG. 2,the fractal heat exchange element has multiple motifs, starting with thelarge triangle at 210, to progressively smaller triangles at 220 and230. However, note that the fractal does not keep extending infinitelyand there are no triangles smaller than the one at 230. In other words,the fractal heatsink 210 has multiple recursive fractal iterations 220and 230, but the fractal iterations stop at level 230 for simplicity ofdesign and manufacturability. Also note that the fractal submotifs 220and 230 are of different dimensional sizes from the original fractalmotif 210 and protrude from the original fractal shape 210. Here, thefirst motif is a large triangle, and the latter motifs are smallertriangles, which involve a rotation, linear displacement, and change ofscale of the prior motif. In one embodiment, the fractal shape has someapertures in it (not illustrated) to allow the solid to be cooled toconnect with other elements. Also, the solid to be cooled is connectedto the fractal shape at point connector 240 and through bus wires at 250and 260. The solid should be connected to the fractal heatsink in atleast one point, either through a point connection, a bus wireconnection, or some other connection. If it is desired that the solid befixed inside the heatsink, there may be at least three connectionpoints, as illustrated. However, only one connection point is necessaryfor conduction between the solid to be cooled and the heatsink.Preferably, the point or bus wire connection is built using a strongheat conductor, such as carbon nanotubes or a diamond-like coating.

Note that, as in FIG. 1, the fractal structure 210 in FIG. 2 hasmultiple concave regions or cavities. When a turbulently flowing fluidpasses around this fractal heatsink, the concave regions or cavitiessubstantially reduce the narrow band acoustic resonance as compared to aflat or Euclidian structure. This allows for more energy to be availableto for heat transfer.

In yet another embodiment of the invention, the heatsink 210 in FIG. 3could be constructed without the connections at points 240, 250, and260. In one embodiment, a liquid or gas would fill the aperture 270 withthe intent that the liquid or gas surround the solid to be cooled, holdit in place, or suspend it. Preferably, the liquid or gas surroundingthe solid would conduct heat from the solid to the heatsink, which wouldthen cause the heat to exit.

In another embodiment of the invention, the heatsink comprises a heatexchange device which is structurally configured based on a QuadraticKoch Island as illustrated in FIG. 4.

FIG. 5A illustrates a square with dimension x₀ that forms the basis forthe Quadratic Koch Island. FIG. 5B illustrates a Quadratic Koch Islandobtained after application of one fractal on the square. The fractalwith section lengths of l is applied to each side of the square in thefirst iteration. Similarly, after several such iterations, a QuadraticKoch Island as illustrated in FIG. 5C may be obtained.

FIG. 6 illustrates the length of the fractal l_(f) which is the totallength of all the fractal segments. The length of each fractal section,l(n), decreases with each iteration of the fractal. The fractal sectionlength is described by eq. 7.

l(n)=(¼)^(n) x ₀  (7)

where,

x₀ is the length of the side of the original square,

n is the number of iterations

As can be seen from eq. 7, the fractal section length decreases aftereach iteration. When the number of iterations becomes increasinglylarge, the section length tends towards being negligible.

Further, it may be mathematically shown that the overall length L of thefractal may be obtained from eq. 8.

$\begin{matrix}{{L(n)} = {x_{0}\left( {1 + {\frac{2}{3}\left( {1 - \frac{1}{4^{n}}} \right)}} \right)}} & (8)\end{matrix}$

where,

x₀ is the length of the side of the original square,

n is the number of iterations

Similarly, it may be shown that the circumference C of the QuadraticKoch Island can be obtained from eq. 9.

C=4(2^(n) x ₀)  (9)

where,

x₀ is the length of the side of the original square,

n is the number of iterations

It is evident that with each iteration, the circumference C increases.However, the cross-sectional area remains constant at x₀ ² since when afractal area is added the same area is subtracted elsewhere.

In one embodiment, the number of iterations corresponding to theQuadratic Koch Island may be greater than 5. Consequently, the heatexchange device functions as a compact heat exchanger. In other words,the heat exchange device has a large heat transfer area per unitexchanger volume. As a result, several advantages are obtained such as,but not limited to, reduction in space, weight, power requirements andcosts. In another embodiment, the number of iterations corresponding tothe Quadratic Koch Island may be less than or equal to 5. Consequently,the heat exchange device may function as a non-compact heat exchanger.

It may be shown with heat transfer analysis that heat transfer and heattransfer coefficient increase independently of each other with everyapplication of the fractal. Further, the increase may be double, orgreater, with every fractal iteration. In general, the increase in heattransfer follows a trend of 2^(n). Moreover, pumping power increases atalmost one and a half the rate. Pumping power is the power needed topump the heat transfer fluid through the heat exchange device.

In yet another embodiment of the invention, the heatsink comprises aheat exchange device which is structurally configured based on amodified Koch Snowflake as illustrated in FIG. 7A. The basis forgenerating the modified Snowflake is an equilateral triangle of width was illustrated in FIG. 7B. In the first iteration, two smallerequilateral triangles of width ⅓ of the base width w are added onto twosides of the base triangle. Similarly, by applying a second and a thirditeration, the modified Koch Snowflakes as illustrated in FIG. 7A may beobtained.

The surface area, A_(s)(n), of the modified Koch Snowflake may beobtained from eq. 10.

$\begin{matrix}{{A_{s}(n)} = {{2\left( {{wt} + {\frac{\sqrt{3}}{4}w^{2}}} \right)} + {\sum_{1}^{n}{\left\lbrack {{\left( \frac{w}{3^{n}} \right)^{2}\left( \frac{\sqrt{3}}{2} \right)} + {\left( \frac{w}{3^{n}} \right)t}} \right\rbrack 2^{{2n} - 1}}}}} & (10)\end{matrix}$

where,

w is the width of the base triangle

n is the number of iterations

t is the thickness of the modified Koch Snowflake

It is evident that the surface area of the modified Koch Snowflakeincreases with each iteration. More specifically, it may be observedthat after 5 iterations there is an increase in surface area of about58%.

Further, the mass of the modified Koch Snowflake may be obtained usingeq. 11.

$\begin{matrix}{{m(n)} = {\left\{ {{\frac{\sqrt{3}}{4}w^{2}} + {\sum_{1}^{n}{\left\lbrack {\left( \frac{w}{3^{n}} \right)^{2}\left( \frac{\sqrt{3}}{4} \right)} \right\rbrack 2^{2n01}}}} \right\} \rho \; t}} & (11)\end{matrix}$

where, w, n, and t are as above, and ρ is the density of the materialmaking up the modified Koch Snowflake.

It may be observed that the change in surface area with respect to thebaseline case (i.e., n=0) is a function of width (w) and thickness (t).However, the change in mass with respect to the baseline is dependent onthe fractal geometry chosen. The mass of the modified Koch Snowflakeincreases with each iteration. However, it converges to a maximum valueof mass increase of approximately 40%.

A heat transfer effectiveness (ε) of the modified Koch Snowflake may bedefined as the ratio of heat transfer achieved to heat transfer thatwould occur if the modified Koch Snowflake was not present. ε may becalculated from eq. 13.

$\begin{matrix}{ɛ = \frac{Q_{c}}{h{A_{b}\left( {T_{b} - T_{\infty}} \right)}}} & (13)\end{matrix}$

where,

Q is the heat rate

h is the heat transfer co-efficient

A is the area

T is the temperature

Further, a heat transfer efficiency (η) of the modified Koch Snowflakemay be defined as the ratio of heat transfer achieved to the heattransfer that would occur if the entire modified Koch Snowflake was atthe base temperature. η may be calculated from eq. 12.

$\begin{matrix}{\eta = \frac{Q_{c}}{h{A_{s}\left( {T_{b} - T_{\infty}} \right)}}} & (12)\end{matrix}$

where, Q, h, A, and T are as above.

The heat transfer effectiveness (ε) increases with each iteration. In anembodiment, the modified Koch Snowflake corresponding to threeiterations may be used to form the heat exchange device. Accordingly, inthis case, the heat transfer effectiveness (ε) may increase by up to44.8%. Further, the increase in heat transfer effectiveness (ε) per massmay be up to 6%. In one embodiment, the material used to make themodified Koch Snowflake may be aluminum. Consequently, heat transfereffectiveness (ε) per mass of approximately two times larger than thatobtained using copper may be achieved.

Further, the heat transfer effectiveness (ε) per mass depends on thethickness of the modified Koch Snowflake. In an embodiment, the ratio ofwidth (w) to thickness (t) corresponding to the modified Koch Snowflakemay be 8. Accordingly, an increase in heat transfer effectiveness (ε)per mass of up to 303% may be achieved at the fourth iteration.

In yet another embodiment of the invention, the heatsink comprises aheat exchange device which is structurally configured based on aSierpinski Carpet as illustrated in FIG. 8A. The Sierpinski Carpet isformed by iteratively removing material from a base geometry such as,but not limited to, a square as illustrated in FIG. 8B. In the firstiteration, a square with ⅓ of the base width (w) is removed. Similarly,by performing second and third iterations, the Sierpinski Carpets asillustrated in FIG. 8A may be obtained.

The surface area, A_(s)(n), of the Sierpinski Carpet may be obtainedfrom eq. 13.

$\begin{matrix}{{A_{s}(n)} = {{2w^{2}} + {3wt} - {\sum_{1}^{n}{8^{n - 1}\left\lbrack {{2\left( \frac{w}{3^{n}} \right)^{2}} - {4\left( \frac{w}{3^{n}} \right)t}} \right\rbrack}}}} & (13)\end{matrix}$

where,

w is the width of the base square

n is the number of iterations

t is the thickness of the Sierpinski Carpet

Starting from n=0, with each subsequent iteration, the surface area ofthe Sierpinski carpet initially reduces before reaching a minimum.However, after reaching the minimum, the surface area increases witheach subsequent iteration. For example, at a width (w) of 0.0508 m anincrease in surface area of 117% may be obtained after five iterations.Similarly, at a width (w) of 0.0254 m, a surface area increase of 265%may be obtained after five iterations.

Further, the mass of the Sierpinski Carpet may be obtained using eq. 14.

$\begin{matrix}{{m(n)} = {\left\{ {w^{2} - {\sum_{1}^{n}\left\lbrack {8^{n - 1}\left( \frac{w}{3^{n}} \right)^{2}} \right\rbrack}} \right\} \rho t}} & (14)\end{matrix}$

where w, n, and t are as above, and ρ is the density of the materialmaking up the Sierpinski carpet

It may be seen from eq. 11 that with each iteration, the mass of theSierpinski carpet decreases. For example, after five iterations, thereis a reduction of 45% of mass of the Sierpinski carpet.

The heat transfer effectiveness (ε) corresponding to the Sierpinskicarpet increases with each iteration. In an embodiment, the Sierpinskicarpet corresponding to three iterations may be used to form the heatexchange device. Accordingly, in this case, the heat transfereffectiveness (ε) may increase by up to 11.4%. Further, the increase inheat transfer effectiveness (ε) per mass corresponding to the Sierpinskicarpet may be up to 59%. In one embodiment, the material used to makethe Sierpinski carpet may be aluminum. Consequently, heat transfereffectiveness (ε) per mass of approximately two times larger than thatobtained using copper may be achieved.

Further, the heat transfer effectiveness (ε) per mass corresponding tothe Sierpinski carpet depends on the thickness of the corresponding tothe Sierpinski carpet. In an embodiment, the ratio of width (w) tothickness (t) corresponding to the corresponding to the Sierpinskicarpet may be 8. Accordingly, an increase in heat transfer effectiveness(ε) per mass of up to 303% may be achieved at the fourth iteration.

In other embodiments, the heatsink may comprise a heat exchange devicewhich is structurally configured based on, but not limited to, one ormore fractals selected from the group comprising: A “scale 2” and “scale3” Mandelbox; Sierpinski tetrahedron; Fractal pyramid; Dodecahedronfractal; 3D quadratic Koch surface (type 1); 3D quadratic Koch surface(type 2); Jerusalem cube; Icosahedron fractal; Octahedron fractal; VonKoch surface; Menger sponge; 3D H-fractal; and Mandelbulb.

In accordance with an embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a Mandelbox asexemplarily illustrated in FIG. 9. A Mandelbox is a box-like fractalobject that has similar properties as that of the Mandelbrot set. It maybe considered as a map of continuous, locally shape preserving Juliasets. Accordingly, the Mandelbox varies at different locations, sinceeach area uses a Julia set fractal with a unique formula. The Mandelboxmay be obtained by applying eq. 15 repeatedly to every point in space.That point v is part of the Mandelbox if it does not escape to infinity.

v=s*ballFold(r,f*boxFold(v))+c  (15)

where boxFold(v) means for each axis a:

-   -   if v[a]>1 v[a]=2−v[a]    -   else if v[a]<−1 v[a]=−2−v[a]

and ballFold(r, v) means for v's magnitude m:

-   -   if m<r m=m/r²    -   else if m<1 m=1/m

In an instance, using the values of s=2, r=0.5 and f=1 in eq. 12, thestandard Mandelbox may be obtained.

In accordance, with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a Sierpinskitetrahedron. The Sierpinski tetrahedron, also called as tetrix, is athree-dimensional analogue of the Sierpinski triangle. The Sierpinskitetrahedron may be formed by repeatedly shrinking a regular tetrahedronto one half its original height, putting together four copies of thistetrahedron with corners touching, and then repeating the process. Thisis illustrated in FIG. 10 for the first four iterations. The Sierpinskitetrahedron constructed from an initial tetrahedron of side-length L hasthe property that the total surface area remains constant with eachiteration.

The initial surface area of the (iteration-0) tetrahedron of side-lengthL is L²√3. At the next iteration, the side-length is halved and thereare 4 such smaller tetrahedra. Therefore, the total surface area afterthe first iteration may be calculated by eq. 16.

$\begin{matrix}{4{\left( {\left( \frac{L}{2} \right)^{2}\sqrt{3}} \right) = {{4\frac{L^{2}}{4}\sqrt{3}} = {L^{2}\sqrt{3}}}}} & (16)\end{matrix}$

This remains the case after each iteration. Though the surface area ofeach subsequent tetrahedron is ¼ that of the tetrahedron in the previousiteration, there are 4 times as many—thus maintaining a constant totalsurface area. However, the total enclosed volume of the Sierpinskitetrahedron decreases geometrically, with a factor of 0.5, with eachiteration and asymptotically approaches 0 as the number of iterationsincreases.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a dodecaedronfractal. The dodecahedron fractal, also called as dodecahedron flake,may be formed by successive flakes of twenty regular dodecahedrons, asexemplarily illustrated in FIG. 11 for second iteration. Each flake isformed by placing a dodecahedron scaled by 1/(2+φ) in each corner,wherein φ=(1+√5)/2.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on an icosahedronflake, also called as a Sierpinski icosahedron. The icosahedron flakemay be formed by successive flakes of twelve regular icosahedrons, asexemplarily illustrated in FIG. 12 for third iteration. Each flake maybe formed by placing an icosahedron scaled by 1/(2+φ) in each corner,wherein φ=(1+√5)/2.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on an octahedronflake. The octahedron flake, or Sierpinski octahedron, may be formed bysuccessive flakes of six regular octahedrons, as exemplarily illustratedin FIG. 13 for third iteration. Each flake may be formed by placing anoctahedron scaled by ½ in each corner.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a 3DQuadtratic Koch. As exemplarily illustrated in FIG. 14, the 3D QuadraticKoch may be obtained by growing a scaled down version of a triangularpyramid onto the faces of the larger triangular pyramid with eachiteration. FIG. 14 illustrates the first four iterations.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a Jerusalemcube, as exemplarily illustrated in FIG. 15. The Jerusalem cube may beobtained by recursively drilling Greek cross-shaped holes into a cube.The Jerusalem Cube may be constructed as follows:

1. Start with a cube.

2. Cut a cross through each side of the cube, leaving eight cubes (ofrank +1) at the corners of the original cube, as well as twelve smallercubes (of rank +2) centered on the edges of the original cube betweencubes of rank +1.

3. Repeat the process on the cubes of rank 1 and 2.

Each iteration adds eight cubes of rank one and twelve cubes of ranktwo, a twenty-fold increase.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a von Kochsurface, as exemplarily illustrated in FIG. 16. The von Koch surface maybe constructed by starting from an equilateral triangular surface. Inthe first iteration, the midpoints of each side of the equilateraltriangular surface are joined together to form an equilateral triangularbase of a hollow triangular pyramid. This process is repeated with eachiteration.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a Mengersponge, as exemplarily illustrated in FIG. 17. The Menger sponge may beconstructed as follows:

1. Begin with a cube (first image).

2. Divide every face of the cube into 9 squares, like a Rubik's Cube.This will sub-divide the cube into 27 smaller cubes.

3. Remove the smaller cube in the middle of each face, and remove thesmaller cube in the very center of the larger cube, leaving 20 smallercubes (second image). This is a level-1 Menger sponge (resembling a VoidCube).

4. Repeat steps 2 and 3 for each of the remaining smaller cubes, andcontinue to iterate ad infinitum.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a 3D Hfractal, as exemplarily illustrated in FIG. 18. The 3D H fractal isbased on an H-tree which may be constructed by starting with a linesegment of arbitrary length, drawing two shorter segments at rightangles to the first through its endpoints, and continuing in the samevein, reducing (dividing) the length of the line segments drawn at eachstage by √2. Further, by adding line segments on the directionperpendicular to the H tree plane, the 3D H fractal may be obtained.

In accordance with another embodiment, the heatsink may comprise a heatexchange device which is structurally configured based on a Mandelbulb,as exemplarily illustrated in FIG. 19. The Mandelbulb is athree-dimensional analogue of the Mandelbrot set. The Mandelbulb may bedefined as the set of those C in

³ for which the orbit of <0, 0, 0> under the iteration v

v^(n)+c is bounded, where the “nth power” of the vector v=

x, y, z

in

³ is given by eq. 17.

v ^(n) :=r ^(n)

sin(nθ)cos(nϕ, sin(nθ)sin(nϕ), cos(nθ)

  (17)

Where

r=√{square root over (x ² +y ² +z ²)},

ϕ=arctan(y/x)=arg(x+yi), and

θ=arctan(√{square root over (x ² +y ²)}/z)=arccos(z/r).

In accordance with another embodiment of the invention disclosed herein,the heatsink comprises a heat exchange device having a plurality of heatexchange elements which are perforated. As a result, an enhanced heattransfer may be achieved. Additionally, use of perforations may increaseheat transfer by up to a factor of two per pumping power. Further, in aspecific embodiment, the plurality of heat exchange elements may behollow. The combination of hollow heat exchange elements withperforations can result in increases in heat transfer greater than thatof a solid heat exchange element of the same diameter. Additionally,increases in heat transfer per pumping power of up to 20% could beachieved by varying the inclination angle and diameter of theperforations in aligned arrays of the plurality of heat exchangeelements. Furthermore, one or more of the number of perforations andshape of perforations may be configured in order to control the heattransfer. For instance, under natural convection, heat transfer isdirectly proportional to the number of square perforations. In anotherinstance, circular and square perforations may be used to obtain higherNusselt number. Since heat transfer is proportional to Nusselt number,greater heat transfer may be achieved with such an arrangement. In yetanother instance, the Nusselt number corresponding to the plurality ofheat exchange elements may be varied based on one or more of a pitch, ahole diameter, a surface area and flow velocity. In particular, bymodifying the pitch of the perforations, the Nusselt number and henceheat transfer may be increased.

In an embodiment, the heat transfer effectiveness of the plurality ofheat exchange elements may be greater than or equal to a minimum valuesuch that addition of the plurality of heat exchange elements isjustified. As a non-limiting example, the minimum value may be ten.

In another embodiment, a spacing between the plurality of heat exchangeelements is determined based on a height of the plurality of heatexchange elements. In a specific embodiment, for a given heat rate, anoptimal spacing between the plurality of heat exchange elements maydecrease with an increase in height of the plurality of heat exchangeelements.

In yet another embodiment, a shape corresponding to the plurality ofheat exchange elements may be configured to provide enhanced heattransfer. For instance, the plurality of heat exchange elements may befluted. As a result, an increase in heat transfer by up to 9% may beachieved. In another instance, the plurality of heat exchange elementsmay be wavy providing an increase in heat transfer by up to 6%. In oneembodiment, the shape corresponding to the plurality of heat exchangeelements may be triangular, circular, elliptical, rectangular andtrapezoidal. For instance, the plurality of heat exchange elements maybe elliptically annular. Further, an elliptical aspect ratiocorresponding to the plurality of heat exchange elements may be variedin order to obtain greater heat transfer efficiency. As a non-limitingexample, the elliptical aspect ratio may be increased in order to obtainhigher heat transfer efficiency. In another instance, the plurality ofheat exchange elements may be trapezoidal with an optimal aspect numberof 1.5. In yet another instance, the plurality of heat exchange elementsmay be diamond shaped pin fins. Further, the pitch corresponding to theplurality of heat exchange elements may be varied to obtain enhancedheat transfer. For example, the pitch may be varied in proportion to therequired heat transfer coefficient. As a result, increase in heattransfer up to 340% beyond that of flat pin fins may be achieved.

In other embodiments of the invention, the surface geometry of theplurality of heat exchange elements may be varied in order to provideenhanced heat transfer. For instance, square ribs along the plurality ofheat exchange elements may be used. As a result, thermal performance mayincrease by up to 30%. In another instance, diamond shaped surfaceprotrusions may be provided over the plurality of heat exchangeelements. Consequently, thermal performance may be increased by up to38% while also leading to better flow distribution. In yet anotherinstance, grooves may be created on the surfaces of the plurality ofheat exchange elements. As a result, heat transfer could increase by upto 25%. In a further instance, dimples may be placed on the flat base ofthe plurality of heat exchange elements forming a pin fin. Consequently,an increase in heat transfer by up to 8% may be achieved while alsoreducing the friction factor by up to 18%. Further, in an instance,convex shaped dimples may be used to obtain greater heat transfer.

In some other embodiments, an orientation of the plurality of heatexchange elements may be varied in order to enhance heat transfer. Forinstance, in case the number of the plurality of heat exchange elementsis large, the plurality of heat exchange elements may be orientedvertically with respect to the flat base of the plurality of heatexchange elements. In another instance, in case the plurality of heatexchange elements are short with a finning factor of less than 2.7, ahorizontal orientation may be used in order to provide better heattransfer.

In other embodiments, the plurality of heat exchange elements may beconfigured in order to control an amount of heat transfer by radiation.For example, the height of the plurality of heat exchange elements maybe maintained short. As a result, up to 55% of the heat transfer maytake place by radiation. On the other hand, the height of the pluralityof heat exchange elements may be increased in order to reduce the amountof heat transfer by radiation. As another example, the plurality of heatexchange elements may be circular around an annular heat pipe. Further,a ratio of spacing between the plurality of heat exchange elements anddiameter of the plurality of heat exchange elements may be controlled inorder to vary the amount of heat transfer by radiation. For instance,the ratio may be decreased in order to decrease the amount of heattransfer by radiation. Similarly, the ratio may be increased in order toincrease the amount of heat transfer by radiation.

In an embodiment, the number of iterations corresponding to the fractalvariation between respective branches of the plurality of heat exchangeelements may be configured in order to control heat transfer. Forinstance, the number of iterations may be increased in order to obtaingreater heat transfer. However, beyond a certain limit, heat transfermay not be directly proportional to the number of iterations.Additionally, varying the number of iterations may also controldiffusion rate across the surfaces of the plurality of heat exchangeelements based on the fact that diffusion rate is directly proportionalto the number of iterations. However, a certain number of iterationssuch as, but not limited to, four to five iterations, the diffusion ratemay converge.

In another embodiment, a dimension corresponding to the fractalvariation between respective branches of the plurality of heat exchangeelements may be configured in order to control heat transfer. Ingeneral, the heat transfer is directly proportional to the fractaldimension. However, this relationship is valid only till a limitednumber of iterations.

In yet another embodiment, the number of branches corresponding to theplurality of heat exchange elements may be configured to control theheat transfer. Under natural convection, heat transfer effectiveness isfound to be directly proportional to the number of branches. However,after a certain number of branch generations, heat transfereffectiveness saturates. Further, a branching ratio may be configured inorder to obtain minimum resistance to heat conduction and hence greaterheat transfer. In a non-limiting example, a branching ratio of 0.707 or0.7937 may be used.

In another embodiment, heat transfer may be controlled based on thevelocity of fluidic heat exchange medium flowing over the plurality ofheat exchange elements. In general, the heat transfer is directlyproportional to the velocity of fluidic heat exchange medium underforced convection. Additionally, the optimal number of branches requiredto maximize heat transfer has been found to reduce with increase invelocity of fluidic heat exchange medium. Accordingly, under forcedconvection with higher velocity, a smaller number of branches may berequired to achieve a required amount of heat transfer. In anotherembodiment, heat transfer by the plurality of heat exchange elements inthe form of an array of perforated fins may be controlled by varying apumping power. In this case, the heat transfer can be inverselyproportional to the pumping power with small increase for turbulentcross-flow but significant increase for parallel flow.

In accordance with embodiments disclosed herein, the heat sink may bemanufactured using manufacturing techniques such as, but not limited to,injection molding, die casting, extrusion, forging, gravitationalmolding, CNC milling, CNC punching, stamping, wire cut machine and wirecut Electrical Discharge Machining (EDM), additive manufacturing (e.g.,3D printing, 2.5D printing, etc.

In a particular embodiment, the heatsink may be manufactured by amachining processing employing cutting tools and controlled slicingtechniques to construct the plurality of heat exchange elements from asolid block of material such as, but not limited to, copper or aluminum.This technique is preferable to construct the plurality of heat exchangeelements with smaller thickness than is possible by other techniquessuch as extrusion. Advantages of the heatsink manufactured using thistechnique include high aspect ratio, thin fin, low tooling cost, easyand inexpensive to prototype, unidirectional flow and single piececonstruction.

In another embodiment, the heatsink may be manufactured by bendingsheets made of, but not limited to, copper or aluminum into fins to formthe plurality of heat exchange elements. The fins are then bonded to theflat base of the heatsink. This technique allows the flat base and thefins to be made of different materials. Advantages of this manufacturingtechnique include light weight of fins, lower tooling cost and differingmaterials for the flat base and the fins.

In yet another embodiment, the heatsink may be manufactured from sheetsof material such as, but not limited to, copper or aluminum bonded ontothe flat base using one or more of epoxy, soldering and brazing. Thistechnique of manufacturing is suitable for high power application withlow thermal resistance and where forced air cooling is available.

In a further embodiment, the heatsink may be manufactured using diecasting. In this technique, material such as, but not limited to, liquidaluminum is forced under high pressure into re-usable steel molds. Thistechnique is especially suited when the plurality of heat exchangeelements are of complex shapes.

Those skilled in the art will recognize many ways to fabricate theheatsinks described herein. For example, modern three-dimensional laserand liquid printers can create objects such as the heatsinks describedherein with a resolution of features on the order of 16 μm. Also, it ispossible to grow a crystal structure using a recursive growth algorithmor through crystal growth techniques. For example, US 2006/0037177,describes a method of controlling crystal growth to produce fractals orother structures through the use of spectral energy patterns byadjusting the temperature, pressure, and electromagnetic energy to whichthe crystal is exposed. This method might be used to fabricate theheatsinks described herein. For larger heatsinks, such as those intendedto be used in car radiators, traditional manufacturing methods for largeequipment can be adapted to create the fractal structures describedherein.

FIGS. 20-37 illustrate various heatsink designs and proposals, which maybe used in conjunction with various embodiments of the technology. Ingeneral, these provide heat transfer surfaces with large surface area,and in many cases, small terminal features, which can accumulate or trapdust or particles. According to the present technology, the accumulationor dust and/or particles may be reduced by the various means disclosedherein.

In a typical prior heatsink, the energy cost of a fan is considered high(and the penalty of noise also considered high), and therefore lowpressure and modest flow rates are provided, with the flow tending to belinear over a set of plates or vanes. Such flow conditions tend topromote particulate deposition on the heat exchange surfaces. On theother hand, in some cases, the energy cost of the fan and/or noise arenot the critical variables to be minimized. In such cases, high flowrates such as to cause turbulent flow are desirable, since these disruptthe boundary layer and provide a higher heat transfer coefficient, whilealso reducing particulate deposition on the heat exchange surfaces. Asshown e.g., in FIG. 36, a spatial-filled fractal or fractal-like objecthas surfaces with characteristic sizes over a broad range. In thesearchitectures, a heat dissipative structure may be provided in or nearthe geometric center. (The structure may be split approximately in half,and the structure mounted over a heat dissipative structure on asurface). A source of compressed air may be provided blowing in a voidnear the heat dissipative structure, with the air flow exiting thestructure through the fractal like object. According to anotherembodiment, a relatively small compressor pressurizes a plenum, which isperiodically exhausted through one or more nozzles, toward heat transfersurfaces subject to fouling. The compressor may act in parallel to afan, i.e., both run concurrently, and the compressor may be run from thesame motor as the fan. The compressor may have at least two modes ofoperation, one employed when the heat dissipation load permits the heatto be shed based on the fan or convective flows, and thereforepermitting the plenum to be charged to relatively high pressures, andthus produce a high impulse to dislodge dust and debris, and anothermode assumed when heat load is high, and a more continuous flow of lowerpressure air from the compressor assist in heatsink operation. In thisway, maximum air flow is available at peak heat dissipation requirementtimes, and a lower air flow with high peak flow rates is available atlow heat dissipation times. Further, it is noted that vibration of theheat exchange elements of the structure may assist in heat dissipation,especially if movements are macroscopic, and thus are associated withpressure gradients and air flows around the elements.

This document describes in detail illustrative examples of the inventiveapparatus, methods, and articles of manufacture for making and usingfractal heatsinks, along with systems and methods for removing dust andparticles from their surfaces. Neither the specific embodiments of theinvention as a whole, nor those of its features necessarily limit thegeneral principles underlying the invention. The specific featuresdescribed herein may be used in some embodiments, but not in others, inthe various combinations and permutations, without departure from thespirit and scope of the invention as set forth herein. Various physicalarrangements of components and various step sequences also fall withinthe intended scope of the invention. Many additional modifications areintended in the foregoing disclosure, and it will be appreciated bythose of ordinary skill in the art that in some instances some featuresof the invention will be employed in the absence of a corresponding useof other features. The illustrative examples therefore do not limit themetes and bounds of the invention and the legal protection afforded theinvention, which function is carried out by current and future claimsand their equivalents.

What is claimed is:
 1. A heatsink receiving heat from a thermalinterface, comprising: a plurality of heat exchange elements in abranched configuration, having surfaces configured to shed heat; and aparticle dislodging device configured to mechanically disrupt anddecrease an accumulation of particles on the surfaces by generatingimpulses, wherein the plurality of heat exchange elements are subjectedto excitation over a range of frequencies representing differentresonances corresponding to different branches of the heatsink.
 2. Theheatsink system according to claim 1, wherein the particle dislodgingdevice comprises a vibrator configured to vibrate the plurality of heatexchange elements.
 3. The heatsink system according to claim 1, whereinthe particle dislodging device comprises at least one of a piezoelectrictransducer, an electromagnetic transducer, and a rotating motor,configured to induce a vibration in the plurality of heat exchangeelements.
 4. The heatsink system according to claim 1, wherein theparticle dislodging device comprises at least one of a fan and a pump,configured to induce a cyclically time-varying flow of a liquid orgaseous heat exchange media over the plurality of heat exchangeelements.
 5. The heatsink system according to claim 1, wherein theparticle dislodging device comprises at least one of a fan and a pump,configured to induce a time varying flow vector of a liquid or gaseousheat exchange media over the plurality of heat exchange elements.
 6. Theheatsink system according to claim 1, wherein the particle dislodgingcomprises at least one of a bimetal element and a shape memory alloyconfigured to undergo a shape change responsive to temperature.
 7. Theheatsink system according to claim 1, wherein the particle dislodgingdevice comprises an active actuator configured to alter at least onespatial relationship of a first of the plurality of heat exchangeelements with respect to a second of the plurality of heat exchangeelements in response to a control signal.
 8. The heatsink systemaccording to claim 1, wherein the particle dislodging device comprises apassive actuator configured to alter at least one spatial relationshipof a first of the plurality of heat exchange elements with respect to asecond of the plurality of heat exchange elements in response to anenvironmental change.
 9. The heatsink system according to claim 1,wherein the particle dislodging device is controlled by an automatedelectronic processor in dependence on a model of the heatsink system.10. The heatsink system according to claim 1, further comprising atleast two transducers generating vibrations dependent on differentcontrol signals.
 11. The heatsink system according to claim 1, furthercomprising a vibration damper configured to damp vibrations.
 12. Theheatsink system according to claim 1, further comprising: an excitationtransducer configured to generate vibrations in the plurality of heatexchange elements; a feedback transducer configured to detectvibrations; and an electronic control configured to process the detectedvibrations, and produce a control signal for the excitation transducer.13. The heatsink system according to claim 1, wherein the plurality ofheat exchange elements have a plurality of resonant vibrationfrequencies over a frequency range, and the particle dislodging devicecomprises an electrical-vibration transducer and an oscillating signalgenerator, configured to generate vibrations over the frequency range,to vibrate the plurality of heat exchange elements at the plurality ofresonant frequencies.
 14. A heatsink method, comprising: receiving heatfrom a thermal interface; shedding heat from surfaces of a plurality ofheat exchange elements in a branched configuration; and mechanicallydisrupting and decreasing an accumulation of particles on the surfacesby generating impulses with a particle dislodging device, wherein theplurality of heat exchange elements are subjected to excitation over arange of frequencies representing different resonances corresponding todifferent branches of the heatsink.
 15. The method according to claim14, wherein the mechanically disrupting comprises vibrating theplurality of heat exchange elements.
 16. The method according to claim14, further comprising inducing a cyclically time-varying flow of aliquid or gaseous heat exchange media over the plurality of heatexchange elements.
 17. The method according to claim 14, furthercomprising automatically controlling the particle dislodging device independence on a model of the heatsink system.
 18. The method accordingto claim 14, further comprising: detecting vibrations with a feedbacktransducer; and processing the detected vibrations, to produce a controlsignal.
 19. The method according to claim 14, wherein the heat exchangesurface comprises a plurality of heat exchange elements having aplurality of resonant vibration frequencies over a frequency range, andthe particle dislodging device comprises an electrical-vibrationtransducer and an oscillating signal generator, further comprisinggenerating vibrations over the frequency range, to resonate theplurality of heat exchange elements at the plurality of resonantfrequencies.
 20. A heatsink receiving heat from a thermal interface,comprising: a plurality of heat exchange elements in a branchedconfiguration, having surfaces configured to shed heat; and a particledegrading device configured to chemically alter an accumulation ofparticles on the surfaces.